In 1 Thessalonians 2.13, Paul is very thankful to God that when those Thessalonians “received the word of God, which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers.”

Some men heard the gospel – that Jesus Christ, the Son of God, would offer himself as a sacrifice and substitution whereby He would take the punishment for man’s sins, and those who believe in Him would have their sins forgiven and would gain Christ’s righteousness – and were repulsed.  The repulsed ones killed the Lord Jesus, drove Paul and other apostles out, hindered gospel preaching, and worst of all, displeased God (verses 15-16).

But Paul rejoiced that some had the insight, knowledge, understanding, and faith to recognize the truth in the good news of Jesus.  While doubters thought Paul and others were making all this up themselves (the word of men), the Thessalonian church knew the good news was really, truly, actually, factually, accurately and without a doubt from God. 

The church members were the “good soil” where the sowers (Paul and others) had sown seed (Mark 4.1-20); God was making sure the planted seed was growing and working in them.  So Paul’s thanksgiving is appropriately to God, who speaks the truth and assures the kingdom growth.