Summa contra Gentiles1.61 Chapter LXI. That God is the most pure truth

The foregoing being established it is evident that in God there is pure truth, in which there can be no alloy of falsehood or deception.

For falsehood is incompatible with truth, even as black with white. Now God is not merely true, but is truth itself. Therefore there can be no falsehood in Him.

Moreover. The intellect is not deceived in knowing what a thing is, as neither is the sense about its proper sensible. Now all knowledge of the divine intellect is as the knowledge of one Who knows what a thing is, as was proved above. Therefore it is impossible that there be error, deception or falsehood in the divine knowledge.

Further. The intellect does not err about first principles, whereas it does sometimes about conclusions, to which it proceeds by arguing from first principles. Now the divine intellect is not argumentative or discursive, as we proved above. Therefore there can be no falsehood or deception therein.

Again. The higher a cognitive power is, the more universal and the more comprehensive is its proper object: wherefore that which the sight knows accidentally, the common sense or the imagination apprehends as included in its proper object. Now the power of the divine intellect is absolutely supreme in knowledge. Therefore all things knowable are compared thereto as knowable properly and per se and not accidentally. But Che cognitive power errs not about such things. Therefore it is impossible for the divine intellect to err about any knowable object.

Moreover. An intellectual virtue is a perfection of the intellect in knowing things. Now the intellect cannot, according to an intellectual virtue, speak false, but always speaks true: because to speak true is the good act of the intellect, and it belongs to virtue to perform a good act. Now the divine intellect is more perfect by its nature than the human intellect is by a habit of virtue, for it is in the summit of perfection. It remains, therefore, that falsehood cannot be in the divine intellect.

Further. The knowledge of the human intellect is somewhat caused by things; the result being that man's knowledge is measured by its objects: since the judgment of the intellect is true through being in accordance with things, and not vice versa. Now the divine intellect is the cause of things by its Knowledge. Wherefore His knowledge must needs be the measure of things: even as art is the measure of the products of art, each of which is so far perfect as it accords with art. Hence the divine intellect is compared to things as things to the human intellect. Now falsehood resulting from inequality between man's mind and things is not in things but in the mind. Wherefore if there were not perfect equality between the divine mind and things, falsehood would be in things but not in the divine mind. And yet there is no falsehood in things, because as much as a thing has of being, so much has it of truth. Therefore there is no inequality between the divine intellect and things: nor is any falsehood possible in the divine mind.

Again. As the true is the good of the intellect, so is falsehood its evil: for we naturally desire to know the true and shun to be deceived by the false. Now evil cannot be in God, as was proved above. Therefore falsehood cannot be in Him.

Hence it is said (Rom. iii. 4): But God is true: and (Num. xxxiii. 19): God is not as a man, that He should lie: and (1 Jo. i. 5): God is light, and in Him there is no darkness.