The message which the Thessalonians received is described as the word of God, which you heard from us.
This is a clumsy phrase in Greek, which could be translated literally as the word of hearing from us of God.
The meaning, however, is fairly clear: the tradition which Paul passed on to the Thessalonians came through human beings (from us
), but was nevertheless the 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.