From a strict grammatical point of view, the relative pronoun translated which
could refer either to the word, or to God (meaning either that the word is at work, or that God is at work). However, the flow of the sentence places the focus very strongly on the word, rather than on God, so it is better to take this clause as a comment on the word.1,2 Of course, this does not mean that the word had a power independent 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.