Summa contra Gentiles1.60 Chapter LX. That God is truth

It follows from what has been said that God Himself is truth.

For truth is a perfection of the intelligence or intellectual operation, as stated above. Now God's act of intelligence is His substance: and since this very act of intelligence is God's being, as we have shown, it is not made perfect by some additional perfection, but is perfect in itself, just as we have said about the divine being. It remains therefore that the divine substance is truth itself.

Again. Truth is a good of the intellect, according to the Philosopher. Now God is His own goodness, as we have shown. Therefore He is also His own truth.

Further. Nothing can be said participatively of God: since He is His own being which participates nothing. Now truth is in God, as was shown above. If, then, it be not said of Him participatively, it follows that it is said essentially. Therefore God is His own truth.

Moreover. Although properly speaking the true is not in things but in the mind, according to the Philosopher, nevertheless sometimes a thing is said to be true, in so far as it attains to the act of its own nature. Hence, Avicenna says in his Metaphysics that the truth of a thing is a property of the nature immutably attached to it, in so far as that thing is naturally inclined to cause a true estimate of itself, and reflects the type of itself that is in the divine mind. Now God is His own essence. Therefore, whether we speak of the truth of the mind, or of the truth of the thing, God is His own truth.

This is confirmed by the authority of our Lord, Who says of Himself (Jo. xiv. 6): I am the way, the truth, and the life.