Jesus explains, ‘What I say to you in the dark…’ What does he mean with the word ‘dark’? In Matthew’s Gospel we hear this word only twice. Here and in 4:16. There Matthew is citing verses from the prophet Isaiah (9:1), ‘The people that lived in darkness have seen a great light.’ Is Jesus speaking to the Twelve about the ‘darkness’ of the people in unbelief?
Or is he simply speaking about the secrecy of the heart? What I say to you in the silence of your prayer… This interpretation makes sense as well. In that case Jesus is emphasizing that our Christian life and the announcement of the Gospel have to be based upon the inspiration of our prayer; upon the personal relation I have with him… or better: upon the relation he has with me.
Source: Jesuits in Britain