For (1063)(gar) is a conjunction which introduces an explanation and in the present context explains that the the clear evidence of the Thessalonians’ acceptance of the Gospel as the Word of God and that Word performing its supernatural work in their hearts (note 1Thess 2:13) is demonstrated by their willingness to endure sufferings for the sake of the Gospel. Their willingness to suffer for the Gospel is added authentication of the veracity of their conversion to God from idols.

The Word (the Gospel) was operative in their lives as demonstrated by their imitation of other believing churches in Judea.

Brethren (80) (adelphos from collative a = denoting unity + delphús = womb) is literally one born from same womb and so a male having the same father and mother as reference person.

Figuratively, adelphos as in this verse refers to a  close associate of a group of persons having well-defined membership, specifically here referring to fellow believers in Christ who are united by the bond of affection and the "brotherhood of suffering"

The term brethren appears nineteen times in 1 Thessalonians (more than any other epistle except 1 Corinthians) and is employed generically, referring to both male and female believers who, like Paul, have been adopted into the eternal family of God. In other contexts brethren can refer to those of the same nationality but not necessarily believers as Peter does in Acts 3...

And now, brethren, I know that you acted in ignorance, just as your rulers did also. (Acts 3:17)

Spurgeon calls our attention to these...

new converts exhibiting the family likeness, turning out to be very like the believers of older churches. Born many miles away from Judaea, with a sea dividing them from the first country where the gospel was preached, yet these Thessalonian Gentiles, when converted, looked wonderfully like the converts from among the Jews . (Sermon)

Became (1096)(ginomai) means to come into existence.

Spurgeon writes...

I only call your attention to the fact that the apostle says, "Ye, brethren, became followers of the churches of God which in Judaea are in Christ Jesus." Here are people converted in Judaea, and they are of a strongly Jewish type; quite another set of people over at Thessalonica become converted to Christ, and though they are thoroughly of the Greek type, they are very like the converts in Judaea. They know nothing about the law of Moses, they have been heathens, worshipping idols; and yet, when they are converted, the strange thing is, that they are exceedingly like those Jews over yonder, to whom idolatry was an abomination. Greek believers are like Hebrew believers. They have never spoken to one another, and nobody has been there to tell them the peculiarities of Christians, and yet a family likeness is distinctly visible. Were you never startled with this, that if, in the preaching of the gospel to-day, we were to bring to the Lord Jesus a person of high rank, and another of the very lowest extraction, they have the same experience, and upon the greatest of subjects they talk in the same way? "Oh, but," you say, "they pick up certain phrases." No, no! They differ in speech: the likeness is in heart and character. I frequently meet with converts who have not attended this place of worship more than half-a-dozen times, but they have been converted, and when they come to tell the story of their inner life you would suppose that they had been born and bred among us, and had learned all our ways; for, though they do not use the phrases which we use, yet they say the same things. The fact is, we are all alike lost and ruined, and we are born again in the same way, and we find the Savior in the same way, and we rejoice in him when we do find him after much the same fashion, and express ourselves very much after the same style. Believers differ in many things, and yet they are alike in the main things. There are no two exactly alike in all the family of God, and yet the likeness to the Elder Brother is to be seen more or less in each one.

It is to me one of the evidences of the truth and divine nature of the work of grace in the heart, that if you take a Hottentot in his kraal, and he is converted, and you take a university man, who has won all the degrees of learning, and he is converted, yet you would not know Sambo from the Doctor when they begin to talk about the things of God. The Hottentot's English may be broken, but his theology is sound. The uneducated man's words may limp, but his heart will leap. Ruin, redemption, and regeneration are the chief subjects in every case. When I am talking sometimes with young converts, and they put their statements oddly and ignorantly, I am reminded of Father Taylor, when he was getting old. The old man sometimes lost the thread of his discourse, and whenever he did so, he used to say, "There, I cannot find the end of that sentence, but I am bound for the kingdom! Brethren, I am bound for the kingdom!" Off he went to something else; for though he could not complete the paragraph he was bound for the kingdom. Some brethren and sisters cannot see to the end of their own experience, but they are bound for the kingdom. They cannot put this and that together to make it ship-shape: but you can see that they are bound for the kingdom. There is the same tear of repentance, the same glance of faith, the same thrill of joy, the same song of confidence: each one according to his measure enjoys the same life, if he is indeed bound for the kingdom. The babe is like the man, and the man reminds you of the babe. We are one spirit in Christ Jesus.

I will not enlarge, except to say that it makes us sing for joy when we can see in ourselves a likeness to the children of God. We, too, resemble the early saints in our experiences. Opposition and tribulation come to us in our measure as they did to them. There are the same afflictions, the same persecutions, the same trials, wherever the work of Christ goes on; but there is the same mighty God to carry on the work of grace, and the same promises of grace to be fulfilled to every believer.  (Sermon)

You brethren became imitators - In chapter they had become imitators also, Paul recording...

You also became imitators of us and of the Lord, having received the word in much tribulation with the joy of the Holy Spirit (see note 1Thessalonians 1:6)

Obviously this imitation was hardly their choice but instead was a reflection of that the power of the gospel had worked itself out in their lives so that they were willing to suffer for the gospel. This was striking proof of the energizing power of the gospel in their lives and it clearly demonstrated that they were not among the superficial hearers Jesus described...

And the one on whom seed (the Word of God) was sown on the rocky places, this is the man who hears the word, and immediately receives (lambano) it with joy  yet he has no firm root in himself, but is only temporary, and when affliction or persecution arises because of the word, immediately he falls away. (Mt 13:20-21)

And those on the rocky soil are those who, when they hear, receive (dechomai) the word with joy; and these have no firm root; they believe for a while, and in time of temptation fall away. (Luke 8:13)

Persecution for Christ's sake did not cause the Thessalonians to fall away.

Imitators (3402) (mimetes) means one who follows. Mimetes basically means to copy or imitate someone's behavior and has many related words in English - "mime" (one who acts out an imitation of another person or animal), "pantomime" (a theater production which originally was without words), "mimeograph" (a machine which makes many copies from one stencil).

Barclay wrote that

When Paul talked of imitation he was using language which the wise men of Greece could understand. Mimesis, imitation, was a main part in the training of an orator. The teachers of rhetoric declared that the learning of oratory depended on three things-theory, imitation and practice. The main part of their training was the study and the imitation of the masters who had gone before. It is as if Paul said: "If you were to train to be an orator, you would be told to imitate the masters of speech. Since you are training in life, you must imitate the Lord of all good life." (cp notes 1 Peter 2:21) (Barclay, W: The Daily Study Bible Series, Rev. ed. Philadelphia: The Westminster Press or Logos)

Churches (1577) (ekklesia from ek = out + kaleo = call) is literally the "called-out ones". Greeks used ekklesia for an assembly of citizens "called out" to transact city business. The church is not an organization but a living organism, Christ's body, composed of individual members (believers) joined together and in and through which Christ, the Head works, carries out His purposes and lives.

In Christ Jesus (see related topic In Christ) - this phrase usually denotes the fellowship which binds together believers but here is used of that same union which binds Christian churches so that their mutual life is caught up into, and sustained from, the life of the risen Christ.

Hiebert has an interesting thought regarding the phrase in Christ Jesus commenting that...

 It adds the spiritual element that distinguishes these assemblies from the Jewish synagogues. The difference between the Jewish synagogues and the Christian assemblies hinges on the acceptance of Jesus as Messiah. The Jews professed to believe God's Word and claimed to be God's assemblies, but when they rejected the Lord Jesus as their Messiah, who came in fulfillment of the promises in God's Word, they showed that they did not believe God's Word. It is the acceptance of Jesus as Messiah that constitutes the vital bond uniting all true Christians. The converts' faith had brought them into vital union with Him; in Him their spiritual life had its source and center. (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Vine rightly reminds us in the day of a plethora of denominations that...

 the measure of their realization of the strength of this spiritual bond may be gauged by the character of the fellowship with Judaean Christians shown later by the church at Thessalonica, see 2 Corinthians 8:14. Churches are knit together not by any external bond, as of order, organization, history, or distinctive doctrine but by the vital relation of each to the one Lord of all, on Whom each is directly dependent, and to Whom alone each is directly responsible. (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson or Logos)

That are in Judea - Are is the verb eimi which in this phrase could be more literally rendered "the being or existing churches". The idea conveyed by this phrase would be  that they were still standing despite the storms of persecution, that they had prevailed against the gates of Hades and thus the work of God had not come to an end in the place of its origin and the home of its fiercest enemies. The conclusion? In the same way the persecution would avail as little at "first Baptist Church" of Thessalonica. It is interesting to recall that the writer himself (Paul) had himself persecuted the church at Jerusalem, writing to the Corinthians...

For I am the least of the apostles, who am not fit to be called an apostle, because I persecuted the church of God. (1 Corinthians 15:9)

Compare Luke's record...

But Saul began ravaging the church, entering house after house; and dragging off men and women, he would put them in prison. (Acts 8:3)

FOR YOU ALSO ENDURED THE SAME SUFFERINGS AT THE HANDS OF YOUR OWN COUNTRYMEN, EVEN AS THEY DID FROM THE JEWS: hoti ta auta epathete (2PAAI) kai humeis hupo ton idion sumphuleton kathos kai autoi hupo ton Ioudaion: (1Thes 3:4; Acts 17:1-8,13; 2 Corinthians 8:1,2) (Acts 8:1,3; 9:1,13; 11:19; 12:1-3; Hebrews 5:7,8; 10:33,34)

For (hoti) can be translated because and here presents the evidence that the Thessalonian believers had become imitators of the Judean churches. The saints in Judea suffered at the hands of the Jews, and the saints in Thessalonica suffered at the hands of the Gentiles, but even this Gentile persecution was encouraged by the Jewish unbelievers (Acts 17:5, 13). Jesus promised that this would happen (John 15:18-27).

Don't forget the intimate association with the acceptance of the word as the Word of God which energizes us as we believe it (and obey it for if we believe it we will obey it). If we are going to experience victory in sufferings, we must appreciate and appropriate the Living Word.

The same sufferings - could also be rendered "fellow sufferings". This is the very idea inherent in the English word sympathy which is derived from sun (with) plus pathos (feelings, emotion, experience) (pathos is etymologically related to the verb below - pascho - to experience or to suffer). With this background one can better understand why sympathy sums up the relationship between the two churches, for as Webster says sympathy is an affinity, association, or relationship between persons or things wherein whatever affects one similarly affects the other. Sympathy represents  the act or capacity of entering into or sharing the feelings or interests of another. In short, sympathy pictures the relationship existing between these churches that are naturally (supernaturally) drawn together. Fellow suffering always forges a strong bond of unity and in the present scenario brought together the hearts of Jews (Jerusalem church) and Gentiles (Thessalonian church) both united in Christ Jesus and the fellowship of His sufferings...

that I may know Him, and the power of His resurrection and the fellowship of His sufferings (pathema = the actual suffering itself, the very pain that one is experiencing right now) being conformed to His death (see note Philippians 3:10) (Comment: The tense of know here suggests "come to know Him." Even though we already know Christ as Savior, we also need to know Him in both the power of His resurrection (see notes Romans 6:11; 6:12; 6:13; Colossians 3:1) and the fellowship of His sufferings - see notes Philippians 1:29; 1 Peter 4:13).

Vine adds that...

Churches are knit together not by any external bond, as of order, organization, history, or distinctive doctrine but by the vital relation of each to the one Lord of all, on Whom each is directly dependent, and to Whom alone each is directly responsible. (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson or Logos)

Endured suffering (3958)(pascho) means to undergo an experience, usually difficult, normally with the implication of physical or psychological suffering.

The writer of Hebrews uses pascho to describe  our Lord's sufferings...

For since He Himself was tempted in that which He has suffered (pascho), He is able to come to the aid of those who are tempted. (see note Hebrews 2:18)

Although He was a Son, He learned obedience from the things which He suffered (pascho). (see note Hebrews 5:8)

Otherwise, He would have needed to suffer often since the foundation of the world; but now once at the consummation of the ages He has been manifested to put away sin by the sacrifice of Himself. (see note Hebrews 9:26)

Therefore Jesus also, that He might sanctify the people through His own blood, suffered outside the gate. (see note Hebrews 13:21)

Peter also uses pascho of Jesus' sufferings writing...

For you have been called for this purpose, since Christ also suffered for (substitutionary atonement implied) you, leaving you an example for you to follow in His steps (see note 1 Peter 2:21) (Comment: The Thessalonians and the churches in Judea were follow their Lord's example.)

Therefore, since Christ has suffered in the flesh, arm yourselves also with the same purpose, because he who has suffered in the flesh has ceased from sin (see note 1 Peter 4 :1)

When the Thessalonians accepted Jesus as Lord, the implication is that they in effect rejected the claims to sovereignty of Caesar as "Lord" along with the tenets of the imperial cult, and thus they were perceived as threats to the established social order and government.

Bruce commenting on their imitation writes that...

In 1:6 the Thessalonians are commended for imitating the missionaries, not least by becoming missionaries in their turn: this was a token of the genuineness of their faith. Now a further token of the genuineness of their faith is said to be their imitation of the Judean churches. But this was not a deliberate imitation they knew of the Judean churches mostly by hearsay rather, the experience of the Judean churches was reproduced in the Thessalonian church. This was no merely external resemblance. Persecution, according to the NT, is a natural concomitant of Christian faith, and for the believers in Thessalonica to undergo suffering for Christ's sake proves that they are fellow-members of the same body as the Judean churches. (Bruce, F F: 1 and 2 Thessalonians. Word Biblical Commentary. Dallas: Word, Incorporated. 1982 or Logos)

Countrymen (4853)(sumphuletes from sún = together with, + phulétes = one of the same tribe from phule = a race, clan or tribe) describes one of the same tribe or fraternity. In the NT, generally a fellow citizen, fellow countryman and in this context countrymen denotes that the persecutors were Gentiles, as indicated the sharp contrast with the Jews as well as by the use of your own.

In Acts 17 we read of persecution although these were doubtless also Jewish in addition to Gentile protagonists...

But the Jews, becoming jealous and taking along some wicked men from the market place, formed a mob and set the city in an uproar; and coming upon the house of Jason, they were seeking to bring them out to the people. 6 And when they did not find them, they began dragging Jason and some brethren before the city authorities, shouting, "These men who have upset the world have come here also; 7 and Jason has welcomed them, and they all act contrary to the decrees of Caesar, saying that there is another king, Jesus." 8 And they stirred up the crowd and the city authorities who heard these things. (Acts 17:5-8)

The Jewish protagonists made a wily appeal to political passions ("another king" in verse 7) and thus had aroused the Gentiles to attack Paul and his colleagues. The result was the persecution of the church at Thessalonica, which had not yet subsided.

Jews (2453) (Ioudaios) is the  the ethnic name of a person who belonged to the Jewish nation

Hiebert writes that...

The fires of persecution against the church were ignited by the unbelieving Jews in Judea; the story of Acts makes it clear that the unbelieving Jews of the dispersion kept those fires burning in the Gentile world. The remark of Tertullian fits the experience of the early churches: "The synagogues of the Jews, founts of persecution." (Hiebert, D. Edmond: 1 & 2 Thessalonians: BMH Book. 1996)

Even as they did from the Jews (2453) (Ioudaios) - this refers of course to the churches in Jerusalem and Judea which had suffered at the hands of the Jews their own countrymen. Such persecution from countrymen is reminiscent of Jesus' prophetic words in Matthew...

"For I came to SET A MAN AGAINST HIS FATHER, AND A DAUGHTER AGAINST HER MOTHER, AND A DAUGHTER-IN-LAW AGAINST HER MOTHER-IN-LAW 36 and A MAN'S ENEMIES WILL BE THE MEMBERS OF HIS HOUSEHOLD. 37 "He who loves father or mother more than Me is not worthy of Me; and he who loves son or daughter more than Me is not worthy of Me. 38 "And he who does not take his cross and follow after Me is not worthy of Me. 39 "He who has found his life shall lose it, and he who has lost his life for My sake shall find it. 40 "He who receives you receives Me, and he who receives Me receives Him who sent Me. (Matthew 10:35-40)

Compare to Micah's charge against his fellow countrymen...