The fool in his heart...

Now it is of course true that this movement of Being between ground and manifestation is not rounded and complete itself, but rather occurs only in the straining of beings beyond themselves; something becomes true in itself only in so far as it is true for others, and something becomes good and worthy in itself in so far as it is worthy of being striven for by others.

-- von Balthasar, "The Glory of the Lord"

A somewhat simple matter. The psalmists of the Old Testament are everywhere addressing God. In all situations, through every emotion, their focus and attention is directed towards the LORD God. Not so for those born from the modern age. These have been trained to relate primarily to their own selves in the passage through their lives.

So in fact, the fool who has said in his heart, 'there is no God' may not be an out-and-out atheist. Rather he is one who is so pre-occupied with relating to himself that God is ignored and considered of no account. The fool is ignorant of God.