For the people of those regions report about us what kind of welcome we had among you, and how you turned to God from idols, to serve a living and true God. (1 Thessalonians 1:9, NRSV)
For they themselves shew of us what manner of entering in we had unto you, and how ye turned to God from idols to serve the living and true God. (1 Thessalonians 1:9, KJV)
Paul praises the Thessalonians for ditching their idol-worshipping ways to follow Jesus. So that's one part of Christian identity down. No idol worship. Check.
For we know, brothers and sisters beloved by God, that he has chosen you, because our message of the gospel came to you not in word only, but also in power and in the Holy Spirit and with full conviction. (1 Thessalonians 1:4-5, NRSV)
Knowing, brethren beloved, your election of God. For our gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance. (1 Thessalonians 1:4-5, KJV)
Another big part of Christian identity is this idea of chosenness. God selected these fine Thessalonian people to be his followers. How does Paul know? Because these guys believed him and have the Holy Spirit. Duh.
You remember our labor and toil, brothers and sisters; we worked night and day, so that we might not burden any of you while we proclaimed to you the gospel of God. You are witnesses, and God also, how pure, upright, and blameless our conduct was toward you believers. As you know, we dealt with each one of you like a father with his children, urging and encouraging you and pleading that you lead a life worthy of God, who calls you into his own kingdom and glory. (1 Thessalonians 2:9-12, NRSV)
For ye remember, brethren, our labour and travail: for labouring night and day, because we would not be chargeable unto any of you, we preached unto you the gospel of God. Ye are witnesses, and God also, how holily and justly and unblameably we behaved ourselves among you that believe: As ye know how we exhorted and comforted and charged every one of you, as a father doth his children, That ye would walk worthy of God, who hath called you unto his kingdom and glory. (1 Thessalonians 2:9-12, KJV)
Paul's own identity is also a big deal here. He sees himself as essentially a hard worker. He's not trying to lead anyone astray or mooch off his congregation. Not only that, but Paul treats these "kids" like he's their dad. Taking care of them, nurturing them, and making sure they don't stay out past curfew.
We also constantly give thanks to God for this, that when you received the word of God that you heard from us, you accepted it not as a human word but as what it really is, God's word, which is also at work in you believers. (1 Thessalonians 2:13, NRSV)
For this cause also thank we God without ceasing, because, when ye received the word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the word of God, which effectually worketh also in you that believe. (1 Thessalonians 2:13, KJV)
Another part of Christian identity: being able to identify God's messengers. The Thessalonians are good to go because they knew that Paul and company were coming with the word of God.
Finally, brothers and sisters, we ask and urge you in the Lord Jesus that, as you learned from us how you ought to live and to please God (as, in fact, you are doing), you should do so more and more. For you know what instructions we gave you through the Lord Jesus. For this is the will of God, your sanctification: that you abstain from fornication; that each one of you know how to control your own body in holiness and honor, not with lustful passion, like the Gentiles who do not know God. (1 Thessalonians 4:1-5, NRSV)
Furthermore then we beseech you, brethren, and exhort you by the Lord Jesus, that as ye have received of us how ye ought to walk and to please God, so ye would abound more and more. For ye know what commandments we gave you by the Lord Jesus. For this is the will of God, even your sanctification, that ye should abstain from fornication: That every one of you should know how to possess his vessel in sanctification and honour; Not in the lust of concupiscence, even as the Gentiles which know not God. (1 Thessalonians 4:1-5, KJV)
Okay, so Paul is pretty interested in people's sex lives. But seriously, he sees this as an important part of their identity as Christians. It's yet another thing that sets them apart from those idol-worshipping Gentiles.
Aspire to live quietly, to mind your own affairs, and to work with your hands, as we directed you, so that you may behave properly toward outsiders and be dependent on no one. (1 Thessalonians 4:11-12, NRSV)
And that ye study to be quiet, and to do your own business, and to work with your own hands, as we commanded you; That ye may walk honestly toward them that are without, and that ye may have lack of nothing. (1 Thessalonians 4:11-12, KJV)
While Paul is mainly focused on group identity, he's also worried about the way Christians are presenting themselves to the outside world. Basically, he thinks they should just be keeping their head down and not causing a fuss. Paul's not the best at following this advice actually.
You yourselves know very well that the day of the Lord will come like a thief in the night. When they say, "There is peace and security," then sudden destruction will come upon them, as labor pains come upon a pregnant woman, and there will be no escape! But you, beloved, are not in darkness, for that day to surprise you like a thief; for you are all children of light and children of the day; we are not of the night or of darkness. (1 Thessalonians 5:2-5, NRSV)
For yourselves know perfectly that the day of the Lord so cometh as a thief in the night. For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape. But ye, brethren, are not in darkness, that that day should overtake you as a thief. Ye are all the children of light, and the children of the day: we are not of the night, nor of darkness. (1 Thessalonians 5:2-5, KJV)
Another contrast between in and out-group identity. Paul knows that the in-group (Christians) are gonna be watching and waiting for the end of the world, so naturally, they'll be ready when it comes. The out-group (non-believers) are not gonna be so lucky. Hey, you snooze, you lose.
But we must always give thanks to God for you, brothers and sisters beloved by the Lord, because God chose you as the first fruits for salvation through sanctification by the Spirit and through belief in the truth. For this purpose he called you through our proclamation of the good news, so that you may obtain the glory of our Lord Jesus Christ. (2 Thessalonians 2:13-14, NRSV)
But we are bound to give thanks always to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth: Whereunto he called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ. (2 Thessalonians 2:13-14, KJV)
Again with the whole choosing thing. God hand-picked these believers to be saved… the rest of the world hasn't been so lucky.