NOW IF THEIR TRANSGRESSION BE RICHES FOR THE WORLD AND THEIR FAILURE IS RICHES FOR THE GENTILES: ei de to paraptoma auton ploutos kosmou kai to etthema auton ploutos ethnon: (15,33; 9:23; Ephesians 3:8; Colossians 1:27) (25; Isaiah 11:11-16; 12:1-6; 60:1-22; 66:8-20; Micah 4:1,2; 5:7; Zechariah 2:11; Zechariah 8:20-23; Revelation 11:15-19)

Now if their stumbling (their lapse, their transgression) has so enriched the world [at large], and if [Israel's] failure means such riches for the Gentiles, think what an enrichment and greater advantage will follow their full reinstatement! (Amplified)

Transgressions (3900) (paraptoma from parapipto = fall aside, apostatize) is a "false step" and describes a deviation from living according to what has been revealed as the right way to live.

Riches (4149) (ploutos from pletho = fill) can describe wealth, money, possessions, or abundance. Here Paul is obviously not speaking of worldly riches but of spiritual riches.

Failure (2275) (hettema from hettáomai = be overcome) describes a deterioration and objectively as used in this verse pictures a failure or a loss.

God has used even the great transgression of Israel in rejecting her Messiah to accomplish His ultimate purpose of bringing spiritual riches to the world, that is, the Gentiles, just as He had declared to Abraham when He first called Israel to Himself: “In you all the families of the earth shall be blessed” (Ge 12:3). Although Israel failed to witness to the world in righteousness, God caused her to witness to the world, as it were, in unrighteousness. Because the Lord could not use Israel’s faithfulness to bring riches to the Gentiles, He instead used her failure.

HOW MUCH MORE WILL THEIR FULFILLMENT BE (pleroma in v25 also = fullness): poso mallon to pleroma auton:

Fulfillment (4138) (pleroma from pleroo = make full, fill, fill up) describes fullness, a full measure, an abundance or a completion. Pleroma is that which has been filled and thus refers to that which is complete, the completeness of Israel referring here to its return to God at the second Advent of the Messiah when all Israel would be saved (cf Ro 11:26).

But if Israel's rejection of light described above brings "riches", how much more will Israel’s restoration result in rich blessing for all the world! When Israel turns to the Lord at the close of the Great Tribulation, she will become the channel of blessing to the nations. Think of it—the Lord Jesus, personally reigning on the throne of David in Jerusalem, the nation Israel saved, Satan bound, and universal righteousness, peace, and prosperity for one thousand years, “that the residue of men might seek after the Lord, and all the Gentiles” (Ac15:17).

"Much more" when used in a comparison regarding quantity, denotes a greater abundance (as here). Otherwise (Ro 5:9,10,15,17) it denotes a greater certainty. What is Israel's "fulfillment" associated with? Messiah's return and the His establishment of the Millennial kingdom. This is why it is "much more"! Not only that but many think that the 144,000 Jews in Revelation 7 will be evangelists and the great reaping of souls from Gentiles is a result of their strong witness (although Scripture never directly identifies the 144,000 as "witnesses" it seems very plausible).

Earlier in the letter Paul had used another extreme contrast to portray the greatness of God’s love and grace:

“God demonstrates His own love toward us, in that while we were yet sinners, Christ died for us. Much more then, having now been justified by His blood, we shall be saved from the wrath of God through Him. For if while we were enemies, we were reconciled to God through the death of His Son, much more, having been reconciled, we shall be saved by His life” (Rom. 5:8–10).

If a dead Savior could redeem us, how much more can a living Savior sustain us.

By the same logic, if a faithless Israel could bring salvation to the Gentiles, how much greater blessing will her faithfulness bring. The Lord promises that when Israel one day receives the Messiah she rejected, He

“will pour out on the house of David and on the inhabitants of Jerusalem, the Spirit of grace and of supplication, so that they will look on Me whom they have pierced” (Zech 12:10).

“In that day a fountain will be opened for the house of David and for the inhabitants of Jerusalem, for sin and for impurity” (Zech 13:1).

Following that,

“the Lord will be king over all the earth; in that day the Lord will be the only one, and His name the only one. . . . And there will be no more curse, . . . Then it will come about that any who are left of all the nations . . . will go up from year to year to worship the King, the Lord of hosts” (Zech 14:9, 11, 16).

"Apostle of Gentiles" - Jesus explained to the somewhat fearful Ananias so that he would be encouraged to go to Saul (Paul) and help him...

"Go, for he is a chosen instrument of Mine, to bear My name before the Gentiles and kings and the sons of Israel for I will show him how much he must suffer for My name's sake." (Acts 9:15-16)

Paul many times emphasized his ministry to the Gentiles...

"(In his Defense before the Jews he testified) and He (the risen Christ) said to me, 'Go! For I will send you far away to the Gentiles.'" (Acts 22:21)

15 "And I said, 'Who art Thou, Lord?' And the Lord said, 'I am Jesus whom you are persecuting. 16 'But arise, and stand on your feet; for this purpose I have appeared to you, to appoint you a minister and a witness not only to the things which you have seen, but also to the things in which I will appear to you; 17 delivering you from the Jewish people and from the Gentiles, to whom I am sending you, 18 to open their eyes so that they may turn from darkness to light and from the dominion of Satan to God, in order that they may receive forgiveness of sins and an inheritance among those who have been sanctified by faith in Me.' (Acts 26:15-18)

(Paul before the Jewish Council at Jerusalem testified that) "And it was because of a revelation that I went up; and I submitted to them the gospel which I preach among the Gentiles, but I did so in private to those who were of reputation, for fear that I might be running, or had run, in vain...I had been entrusted with the gospel to the uncircumcised, just as Peter had been to the circumcised 8 (for He who effectually worked for Peter in his apostleship to the circumcised effectually worked for me also to the Gentiles), 9 and recognizing the grace that had been given to me, James and Cephas and John, who were reputed to be pillars, gave to me and Barnabas the right hand of fellowship, that we might go to the Gentiles, and they to the circumcised. (Gal 2:2,7-9)

“the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ” (Ephesians 3:8).

And for this (the fact that there was one God and one mediator between God and men, the man Christ Jesus) I was appointed a preacher and an apostle (I am telling the truth, I am not lying) as a teacher of the Gentiles in faith and truth.(1 Timothy 2:7)

Paul understood the sovereignty of God in salvation. He understood that whom God foreknew He predestined and called (Ro 8:28-30) and yet here we see he approaches this issue of salvation with a sense of his responsibility of delivering the gospel to all men for he does not know who the elect are.

We see a similar thought in his last known letter, 2 Timothy, in which he explained that it was because of the gospel that ...

I suffer hardship even to imprisonment as a criminal; but the word of God is not imprisoned.  For this reason I endure all things (man's responsibility) for the sake of those who are chosen (elect - God's sovereignty), that they also may obtain the salvation which is in Christ Jesus and with it eternal glory. (see notes on 2 Timothy 2:9-10)

Note that when Paul first came to a city, he invariably would go first to the synagogue or search out individual Jews if they were too few to have a synagogue. Some of those hearers would believe, but, for the most part, they resisted and rejected the gospel that was “spoken against everywhere” (Acts 28:22). At that point, Paul would turn his focus to the Gentiles.

In this way Paul would "magnify" his "ministry" or as Phillips paraphrases it...

In the hope of making my fellow Jews jealous [in order to stir them up to imitate, copy, and appropriate], and thus managing to save some of them. (Amplified)

If somehow. Elsewhere Paul stated it this was...

And to the Jews I became as a Jew, that I might win Jews; to those who are under the Law, as under the Law, though not being myself under the Law, that I might win those who are under the Law; to those who are without law, as without law, though not being without the law of God but under the law of Christ, that I might win those who are without law. To the weak I became weak, that I might win the weak; I have become all things to all men, that I may by all means save some. (1 Cor 9:20-22)

Jealous (3863) (parazeloo from pará = to the point of, unto, implying movement toward a certain point + zelóo = to desire, be zealous) means to stimulate alongside and thus to excite to rivalry or to provoke to emulation and so to make jealous. The idea is that then the Jews would be jealous and want it for themselves. Parazeloo is a Greek infinitive with a preposition (eis = unto, toward) and carries the idea of purpose.

AND SAVE SOME OF THEM: kai soso (1SFAI) tinas ex auton: (1 Corinthians 7:16; 1 Timothy 4:16; James 5:20)

For if their rejection and exclusion from the benefits of salvation were [overruled] for the reconciliation of a world to God, what will their acceptance and admission mean? [It will be nothing short of] life from the dead! (Amplified)

The word “if” which Paul uses throughout his argument here is not ean the “if” of a hypothetical condition, but ei the “if” of a fulfilled condition. Paul is not arguing upon the basis of an hypothesis, but upon the basis of facts. The translation should read, “since,” or “in view of the fact.”

Rejection (580) (apobole from apó = from + bállo  = cast as casting away a garment) describes literally a casting off and thus a rejection.

Reconciliation (2643) (katallage from katallasso = change mutually; reconcile in turn from katá = an intensifying preposition + allásso = to make otherwise, to change the form or nature of a thing) means a change or reconciliation from a state of enmity between persons to one of friendship. In the NT, this word describes restoration to divine favor by bringing about a change in man, i.e., a conversion, the means or occasion of reconciling the world to God. Reconciliation is the removal of enmity that stands between people and God. Reconciliation is the basis of restored fellowship between people and God

The Greek verb katallasso basically means “change” or “exchange.” In the context of relationships between people, the term implies a change in attitude on the part of both individuals, a change from enmity to friendship. When used to describe the relationship existing between God and a person, the term implies the change of attitude on the part of both a person and God. The need to change the sinful ways of a human being is obvious. It should be noted that some argue that no change is needed on the part of God. On the other hand, inherent in the doctrine of justification is the changed attitude of God toward the sinner. God declares a person who was formerly His enemy to be righteous before Him.

WHAT WILL THEIR ACCEPTANCE BE BUT LIFE FROM THE DEAD: tis e proslempsis ei me zoe ek nekron: (Ezekiel 37:1-14; Revelation 11:11; 20:4-6)

Acceptance (4356) (proslepsis from proslambáno = to receive unto oneself, to take in addition, to receive besides) means a taking to oneself and thus describes an acceptance.

The reconciling of Israel back to God is, indeed, associated with the resurrection of Israel as a nation (Ezekiel 37:1-14), and also with the bodily resurrection of all who have died in faith, Isaiah prophesying to Israel that...

Your dead will live; their corpses will rise. You who lie in the dust, awake and shout for joy, for your dew is as the dew of the dawn, and the earth will give birth to the departed spirits. (Isa 26:19, compare Hosea 6:1-3 Rev 20:4-6)