BUT YOU HAVE COME TO MOUNT ZION AND TO THE CITY OF THE LIVING GOD THE HEAVENLY JERUSALEM: alla proseleluthate (2PRAI) Sion horei kai polei theou zontos (PAPMSG) Ierousalem epouranio:

The writer's point in this section is that instead of returning to Mount Sinai, the Hebrew audience is urged to continue their approach to Mount Zion, the spiritual mountain and city where the living God dwells and reigns.

The Nelson Study Bible nicely summarizes this section noting that...

In these verses, the author of Hebrews contrasts the Mosaic covenant with the New Covenant by contrasting two mountains: Mount Sinai and Mount Zion. At Mount Sinai, the Israelites received the Law from God with fear and trembling, for God displayed at that time His awesome power (see Ex 19:10-20:26). In contrast, Christian believers have come to a heavenly Jerusalem on Mount Zion through Jesus’ blood. This mountain is a celebration of the Holy One, attended by angels, believers, and righteous people. The author makes the contrast between the two covenants vivid, and then once again exhorts his readers not to reject Christ’s offer of salvation (see Hebrews 12:25-29).(Radmacher, E. D., Allen, R. B., & House, H. W. The Nelson Study Bible: NKJV.  Nashville: Thomas Nelson)

Have come (4334) (proserchomai from prós = facing + erchomai = come) means literally to come facing toward and so to approach or come near. To come to visit or associate with. It describes the approach to or entry into a deity’s presence. In the Septuagint (LXX) proserchomai was the verb used to describe the approach of the priests to Jehovah for worship and to perform of their priestly (Levitical) functions. But here in Hebrews, under the New covenant, all seven uses of  proserchomai refer to believers possessing the privilege of access to God the Father through Christ the Great High Priest.

Here are the seven uses of this proserchomai in Hebrews...

Hebrews 4:16 (note) Let us therefore draw near with confidence to the throne of grace, that we may receive mercy and may find grace to help in time of need. (Comment: "Let us..." emphasizes that this privilege is always available to those under the New Covenant. Do we really comprehend and avail ourselves of the profundity of this privilege?)

Hebrews 7:25 (note) Hence, also, He is able to save forever those who draw near (present tense = emphasizes continual activity) to God through Him, since He always lives to make intercession for them.

Hebrews 10:1 (note) For the Law, since it has only a shadow of the good things to come and not the very form of things, can never by the same sacrifices year by year, which they offer continually, make perfect those who draw near.

Hebrews 10:22 (note) let us draw near with a sincere heart in full assurance of faith, having our hearts sprinkled clean from an evil conscience and our bodies washed with pure water.

Hebrews 11:6 (note) And without faith it is impossible to please Him, for he who comes (drawn near) to God must believe that He is, and that He is a rewarder of those who seek Him.

Hebrews 12:18 (note) For you have not come (drawn near) to  a mountain that may be touched and to a blazing fire, and to darkness and gloom and whirlwind,

Hebrews 12:22 (note) But you have come (drawn near) to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to myriads of angels

The writer uses the perfect tense which views this heavenly possession of Mt Zion as already attained by those who have believed the new covenant and which is their continuing possession. In spirit they were residents of the city already, though in body they were strangers and aliens on earth. That there is yet to be an earthly manifestation of the city is clear from the later reference in Hebrews 13:14 to “the city which is yet to come.”

Mount Zion (see notes on Zion) is the site of Mt Moriah  (means "the place where Yahweh sees") where Abraham sacrificed Isaac and corresponds to the Dome of the Rock on Temple Mount. Many scholars feel that the Temple Mount area is in the approximate site of Mt Moriah where Abraham offered Isaac his "only son" whom he loved. Centuries later Mt Moriah was the site of the threshing floor of Ornan which David purchased for Solomon's temple (1Chr 21:18ff, 2Sa 24:24, 25, 2Chr 3:1] And nearby is another "mount" of sacrifice known as Golgotha (Jn19:17) also called Calvary (in same range as Moriah but slightly NW) on which God the Father offered up His only Son, the Son Whom He loved (Jn 3:16, 3:35)

Mount Zion was also the location of the Jebusite stronghold that David captured (2Sa 5:7) and made the religious center of his kingdom by bringing to it the golden Ark of God representing God’s presence with His people. This is the site on which Solomon later built the Temple and installed the Ark (1Ki 8:1).

Zion is synonymous with Jerusalem and thus this holy place became the earthly dwelling-place of God. In the new Covenant, believers in Christ have come to the heavenly counterpart, the Jerusalem from above.

Paul distinguished between the earthly and heavenly Jerusalem in his allegorical exposition on Hagar (and Ishmael, born according to the flesh) and Sarah (and Isaac, born according to the Spirit)...

Now this Hagar is Mount Sinai in Arabia, and corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free; she is our mother. (Galatians 4:25-26) (Comment: The phrase Jerusalem which is above was familiar to the rabbinical teachers, who conceived the heavenly Jerusalem as the archetype of the earthly. On the establishment of Messiah’s kingdom, the heavenly archetype would be let down to earth, and would be the capital of the messianic theocracy.)

And so as depicted allegorically by Paul in Galatians 4, the Law (the Old Covenant) had its Mount Sinai but faith has its Mount Zion which is the same city which Abraham and the patriarchs sought, the writer of Hebrews recording that by faith (not sight) Abraham

was looking for the city which has foundations, Whose Architect and Builder is God... (see note Hebrews 11:10)

But as it is, they desire a better country, that is, a heavenly one. Therefore God is not ashamed to be called their God; for He has prepared a city for them (see note Hebrews 11:16).

John saw this city in his vision recording...

And I saw a new heaven and a new earth; for the first heaven and the first earth passed away, and there is no longer any sea. And I saw the holy city, new Jerusalem, coming down out of heaven from God, made ready as a bride adorned for her husband. (See notes Revelation 21:1; Revelation 21:2)

In one sense, this is still to come (see note Hebrews 13:14, “but we are looking for the city that is to come”), but we have also already arrived there in spirit. In Ephesians for example Paul describes the heavenly position of believers writing that God has...

raised us up with Him (Christ), and seated us with Him (Christ, Who is at the right hand of the throne of God) in the heavenly places, in Christ Jesus (see note Ephesians 2:6)

Christians are now citizens of the heavenly city and enjoy its privileges. Paul wrote that believers are enrolled as citizens of heaven even while on earth Paul explaining that...

our citizenship is in heaven, from which also we eagerly wait for a Savior the Lord Jesus Christ (see note Philippians 3:20)

We are in Mount Zion by virtue of our incorporation in Christ as explained above. To reiterate, the use of the perfect tense (have come) emphasizes our permanent, continuing state. This is why the seemingly endless miles of life’s marathon and the inevitable heartbreak hills should not deter us from pressing on toward the goal, running the race with endurance.. We are both in Zion and yet at the same time are marching to Zion!

Law had its earthly Jerusalem but faith has its heavenly capital above. The city of the living God is in heaven, the city which has the foundations, whose Architect and Builder is God.

As someone has well stated...

We have already arrived in principle where in full reality we shall be forever. The future is already the present. In today we possess tomorrow. On earth we own Heaven.

AND TO MYRIADS OF ANGELS: kai muriasin aggelon:

Myriads (3461)(murias from muríos = very many, innumerable) means a myriad (an indefinitely great number) or ten thousand or simply a number that cannot be counted.

Wuest comments that...

The angels are introduced here because they are the usual accompaniment of God’s glory and ministers of His will.

The myriads of angels are referred to several times in Scripture. For example Moses records a "sighting" with which most of his Jewish readers would have been familiar...

And he said, "The LORD came from Sinai, And dawned on them from Seir; He shone forth from Mount Paran, And He came from the midst of ten thousand holy ones; At His right hand there was flashing lightning for them. (Dt 33:2) (Evidently there were myriads of holy ones with God on Mount Sinai as His right hand wrote the commandments for Israel on tables of stone.

Angels were present at the giving of the law Luke recording...

you who received the law as ordained by angels, and yet did not keep it." (Acts 7:53)

In a parallel passage Paul asks...

Why the Law then? It was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed should come to whom the promise had been made. (Galatians 3:19) (Comment: The account of the giving of the law through Moses on Mount Sinai in Exodus 19:9-25 makes no mention of angels. However as noted above Deuteronomy 33:2 does mention holy ones which is almost assuredly a reference to angels through whom the Law was ordained. See also Psalm 68:17).

And earlier in Hebrews the writer reminded his readers of...

the word spoken through angels (which) proved unalterable, and every transgression and disobedience received a just recompense (see notes Hebrews 2:2)

And as the writer of Hebrews asked rhetorically in the first chapter...

Are they (referring to angels) not all ministering spirits, sent out to render service for the sake of those who will inherit salvation? (See note Hebrews 1:14)

These angels are mighty flaming spirits passing in and out of our lives, moving around us and over us just as they did saints of old.

The Living God - a phrase repeated 28 times in the NASB...

Dt 5:26 'For who is there of all flesh, who has heard the voice of the living God speaking from the midst of the fire, as we have, and lived?

Josh 3:10 And Joshua said, "By this you shall know that the living God is among you, and that He will assuredly dispossess from before you the Canaanite, the Hittite, the Hivite, the Perizzite, the Girgashite, the Amorite, and the Jebusite.

1 Sam 17:26 Then David spoke to the men who were standing by him, saying, "What will be done for the man who kills this Philistine, and takes away the reproach from Israel? For who is this uncircumcised Philistine, that he should taunt the armies of the living God?"

1 Sam 17:36 "Your servant has killed both the lion and the bear; and this uncircumcised Philistine will be like one of them, since he has taunted the armies of the living God."

2 Ki 19:4 'Perhaps the LORD your God will hear all the words of Rabshakeh, whom his master the king of Assyria has sent to reproach the living God, and will rebuke the words which the LORD your God has heard. Therefore, offer a prayer for the remnant that is left.'"

2 Ki 19:16 "Incline Thine ear, O LORD, and hear; open Thine eyes, O LORD, and see; and listen to the words of Sennacherib, which he has sent to reproach the living God.

Psa 42:2 My soul thirsts for God, for the living God; When shall I come and appear before God?

Psa 84:2 My soul longed and even yearned for the courts of the LORD; My heart and my flesh sing for joy to the living God.

Isa 37:4 'Perhaps the LORD your God will hear the words of Rabshakeh, whom his master the king of Assyria has sent to reproach the living God, and will rebuke the words which the LORD your God has heard. Therefore, offer a prayer for the remnant that is left.'"

Isa 37:17 "Incline Thine ear, O LORD, and hear; open Thine eyes, O LORD, and see; and listen to all the words of Sennacherib, who sent them to reproach the living God.

Jer 10:10 But the LORD is the true God; He is the living God and the everlasting King. At His wrath the earth quakes, And the nations cannot endure His indignation.

Jer 23:36 "For you will no longer remember the oracle of the LORD, because every man's own word will become the oracle, and you have perverted the words of the living God, the LORD of hosts, our God.

Dan 6:20 And when he had come near the den to Daniel, he cried out with a troubled voice. The king spoke and said to Daniel, "Daniel, servant of the living God, has your God, whom you constantly serve, been able to deliver you from the lions?"

Dan 6:26 "I make a decree that in all the dominion of my kingdom men are to fear and tremble before the God of Daniel; For He is the living God and enduring forever, And His kingdom is one which will not be destroyed, And His dominion will be forever.

Hosea 1:10 Yet the number of the sons of Israel Will be like the sand of the sea, Which cannot be measured or numbered; And it will come about that, in the place Where it is said to them, "You are not My people," It will be said to them, "You are the sons of the living God."

Mat 16:16 And Simon Peter answered and said, "Thou art the Christ, the Son of the living God."

Mat 26:63 But Jesus kept silent. And the high priest said to Him, "I adjure You by the living God, that You tell us whether You are the Christ, the Son of God."

Acts 14:15 and saying, "Men, why are you doing these things? We are also men of the same nature as you, and preach the gospel to you in order that you should turn from these vain things to a living God, WHO MADE THE HEAVEN AND THE EARTH AND THE SEA, AND ALL THAT IS IN THEM.

Rom 9:26 "AND IT SHALL BE THAT IN THE PLACE WHERE IT WAS SAID TO THEM, 'YOU ARE NOT MY PEOPLE,' THERE THEY SHALL BE CALLED SONS OF THE LIVING GOD."

2 Cor 3:3 being manifested that you are a letter of Christ, cared for by us, written not with ink, but with the Spirit of the living God, not on tablets of stone, but on tablets of human hearts.

2 Cor 6:16 Or what agreement has the temple of God with idols? For we are the temple of the living God; just as God said, "I WILL DWELL IN THEM AND WALK AMONG THEM; AND I WILL BE THEIR GOD, AND THEY SHALL BE MY PEOPLE.

1 Tim 3:15 but in case I am delayed, I write so that you may know how one ought to conduct himself in the household of God, which is the church of the living God, the pillar and support of the truth.

1 Tim 4:10 For it is for this we labor and strive, because we have fixed our hope on the living God, who is the Savior of all men, especially of believers.

Heb 3:12 Take care, brethren, lest there should be in any one of you an evil, unbelieving heart, in falling away from the living God.

Heb 9:14 how much more will the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, cleanse your conscience from dead works to serve the living God?

Heb 10:31 It is a terrifying thing to fall into the hands of the living God.

Heb 12:22 But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to myriads of angels,

Rev 7:2 And I saw another angel ascending from the rising of the sun, having the seal of the living God; and he cried out with a loud voice to the four angels to whom it was granted to harm the earth and the sea,

An illustration - The little girl was happily humming a hymn as she dusted the furniture to help her mother.

“Mommie, will I be dusting God’s chair when I get to heaven, the way the hymn says?” she asked.

Mother looked up with surprise,

“Which hymn, honey?”

“And dust around the throne,” her little girl quoted.

It took a while before the mother learned that she was quoting a line from the hymn “Marching to Zion,” with the phrase, “and thus surround the throne”!

TO THE GENERAL ASSEMBLY AND CHURCH OF THE FIRST-BORN WHO ARE ENROLLED IN HEAVEN: kai ekklesia prototokon apogegrammenon (RPPMPG) en ouranois:

General assembly - note that in the Greek text paneguris is the last section of Hebrews 12:22 (see there) but is placed in verse 23 in the English.

General assembly (3831) (paneguris from pás = all, + águris  = an assembly from agora = public square, marketplace) is used only here in the NT to describe a solemn assembly convened for purposes of a happy, joyous festivity. The Greeks used paneguris to describe a festal gathering of the whole people to celebrate public games or other solemnities.

The verb form panegurizo is used in Isaiah where God declares...

Be joyful with Jerusalem and rejoice for her (Septuagint translates the Hebrew with the verb panegurizo = to celebrate a public festival), all you who love her; Be exceedingly glad with her, all you who mourn over her (Isaiah 66:10)

As explained below Jesus is the firstborn and by virtue of our union with Him we are firstborn sons of God. All the rights of inheritance go to the firstborn—to us who are “co-heirs with Christ” (see note Romans 8:17).

First-born (firstborn) (4416)(prototokos from protos = first, foremost, in place order or time; rank dignity + titko = beget, to bear, bring forth)  can mean first-born chronologically (Lu 2:7), but refers primarily to position, rank, priority of position and emphasizes quality or kind, not time with the idea of "preeminence".

Prototokos is used 8 times in the NT and four refer to Jesus -- Colossians 1:18 (note), Romans 8:29 (note), Hebrews 1:6 (note), and Revelation 1:5 (note) Each of these references to Christ as prototokos are in somewhat different contexts so be careful not to be confused. Vine helps untangle these uses by pointing out that in Colossians 1:18 (note) and Revelation 1:5 (note) firstborn refers to His resurrection, in Romans 8:29 (note) to His position in relationship to the Church, in Hebrews 1:6 (note) to His Second Advent when the word “again” is place in the right place (the Authorized Version gives a wrong translation, making the “again” seem to introduce a quotation, instead of signifying the second time when God will bring His Son into the world).

In Ps 89:27, God says of the Messiah that He "shall make him My first-born" then defines what He means—"the highest of the kings of the earth."

In the present verse the firstborn enjoy the rights of firstborn sons, because of their union with Christ, the Firstborn.

The assembly of the firstborn will have all Christians of all times as its members. It will have its first full assembly in the future age with the OT spirits of righteous men made perfect are together with NT saints forever. And thus this reference is to the whole communion of saints (all Christians of all times) who have come, not merely into the presence of the church, but into its membership by faith in Christ. The writing of their names in heaven recalls Jesus’ words to his disciples,

 Rejoice that your names are written in heaven (Lk10:20).

They will all share with Jesus the title of firstborn (Col 1:18) because they are “heirs of God and coheirs with Christ” (see note Romans 8:17).

AND TO GOD, THE JUDGE OF ALL: kai krite theo panton:

God is not unjust so as to forget your work and the love which you have shown toward His name, in having ministered and in still ministering to the saints. (see note Hebrews 6:10)

there is no creature hidden from His sight, but all things are open and laid bare to the eyes of Him with whom we have to do  (see note Hebrews 4:13).

And for those who reject Jesus' sacrifice and insult His Spirit of grace there remains no hope but instead...

a terrifying expectation of judgment and THE FURY OF A FIRE WHICH WILL CONSUME THE ADVERSARIES (See note Hebrews 10:27)

For we know Him Who said, "VENGEANCE IS MINE, I WILL REPAY." And again, "THE LORD WILL JUDGE HIS PEOPLE." It is a terrifying thing to fall into the hands of the living God.  (see notes Hebrews 10:30; 10:31).

This relief from the fear of judgment is an enormous blessing to those who know themselves to be (saved) sinners in thought, word and deed.

AND TO THE SPIRITS OF RIGHTEOUS MEN MADE PERFECT: kai pneumasi dikaion teteleiomenon (RPPMPG):

See note Hebrews 11:40 (apart from us they should not be made perfect)