That God is Truth

TRUTH is a perfection of the understanding and of its act. But the understanding of God is His substance; and the very act of understanding, as it is the being of God, is perfect as the being of God is perfect, not by any superadded perfection, but by itself. It remains therefore that the divine substance is truth itself.

4. Though truth is properly not in things but in the mind, nevertheless a thing is sometimes called true, inasmuch as it properly attains the actuality of its proper nature. Hence Avicenna says that the truth of a thing is a property of the fixed and appointed being of each thing, inasmuch as such a thing is naturally apt to create a true impression of itself, and inasmuch as it expresses the proper idea of itself in the divine mind.* But God is His own essence: therefore, whether we speak of truth of the intellect or truth of the object, God is His own truth.

This is also confirmed by the authority of our Lord saying of Himself: I am the way and the truth and the life (John xiv, 6).

1.59 : That the Truth to be found in Propositions is not excluded from God
1.61 : That God is Pure Truth