3-4: Paul was thankful to God for the Christians in Thessalonica and even boasted to other churches about their faithfulness, steadfastness, and growing love for one another in the midst of Persecution and affliction.
5-9: God's judgement is righteous and perfect. He will punish all unrepentant sinners when He returns. But those who obey the Gospel by putting their trust in Jesus for the forgiveness of sins will be considered worthy of God's kingdom and escape judgment.
10: God receives glory through the saints (those redeemed by Jesus) because their salvation is a display of his love and grace. We will worship and marvel at Him throughout all of eternity,
11-12: None of us are worthy of God's calling, which is salvation through Jesus. He graciously saves us and powerfully works through us to accomplish His will. Since he is the source of our worthiness and good works, he deserves all the glory for them. Left to ourselves we would ultimately be lost in our sins, pursuing misguided desires.
1-12: Because of the many false claims that the Day of the Lord had already come, Paul assured them it wouldn't come until "the man of lawlessness" was revealed. He was referring to the Antichrist who will claim to be the one true god and demand worship. He will be supernaturally powered by Satan to deceive with miracles. God "sending a delusion" is simply Him letting those who reject Christ pursue their sinful desires and be deceived. It's the type of judgement spoken of in Romans 1.
13-17: God saved us and is giving us eternal comfort so we should stand firm in Christ, holding to the teaching and traditions in scripture.
1-5: Paul requested prayer for protection from the evil men persecuting him and his disciples. He acknowledged that the Thessalonians were committed to God and that God was faithful to them. God establishes us by grounding our salvation in Jesus' righteousness rather than our own. He guards us from Satan by giving us the Holy Spirit to resist evil.
6-12: Christians should be self-sufficient through hard work to honor God. We shouldn't associate with lazy believers who take advantage of others to meet their material needs. Doing so enables them and wastes money that could be given to the poor, the orphans, and the widows. Paul and his disciples served as examples in hard work and self-sufficiency.
13-15: We need to take disobedience of God's word seriously in the Church. When we lovingly admonish fellow believers for their disobedience it helps both parties: They will be convicted of their sin and we won't be tempted by their lifestyle
16-18: Paul's benediction is a request that God give them peace and grace. To show his genuineness he personally signed all his letters. This was necessary because he dictated them to scribes.