The Thessalonians had (objectively) received the word of God from Paul and his coworkers. Paul then points out that their perception and acceptance of the message corresponded to its true nature: they accepted it not as the word of men, but as it actually is—the word of God.
In drawing this contrast, Paul is not denying the human element in his preaching (which reflected his own thinking and understanding), but he is asserting that his message came ultimately from God—as he has been careful to point out (1 Thessalonians 2:2, 1 Thessalonians 2:4, 1 Thessalonians 2:8, 1 Thessalonians 2:9). He is also seeking to affirm the faith of the persecuted believers, and to strengthen them against the temptation of turning away from the Lord Jesus. They must be assured that what they believed was no made-up tale, but the very word of God.
13 And we also thank God constantly for this, that when you 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.