“For this reason we also thank God without ceasing, because when you received the word of God which you heard from us, you welcomed it not as the word of men, but as it is in truth, the word of God, which also effectively works in you who believe.” 1 Thessalonians 2:13 (NKJV)
The apostle had preached the Gospel of Jesus Christ to the people of Thessalonica, and these who were the Church had received it as the word of God, and had welcomed it.
When we receive the Gospel of Christ, we are receiving the word of God, and not as the words of men. What is the difference in the words of men, and the word of God? Anyone who will be honest about it will note that the words of men constantly change, and are about as reliable and trustworthy as a broken dam. The word of God on the other hand is trustworthy, because God never changes; so His word does not change either.
The word of God is something that continually works through the power of the Spirit of God in you and me changing us into the image of God’s Son Jesus; which is where God desires us to be.
Unceasing thanksgiving from Paul, because they had received the word of God. This should also cause us to be thankful, unceasingly, for the word of God which “Effectively works in you who believe.”