Summa summae S. Thomae, Tomus III

The sense of the question is twofold. First : Can God reveal falsehood, or lie ? Second : Can one elicit from divine faith a false assent, or believe something to be true and revealed by God which is not true nor revealed by God ?

I say 1° : God cannot absolutely lie, or say what is false¹, according to that saying of the Apostle, Hebr., VI, 18 : It is impossible for God to lie.

It is proved. It is absolutely impossible for the first and essential truth to lie. But God is the first and essential truth. Therefore. — Proof of the major. To lie is to fall short of the truth. But it involves a contradiction for the first and essential truth to fall short of the truth ; otherwise it would fall short of itself, and thus would not be the first, essential, and unfailing truth.

Confirmed 1°. If God could absolutely lie, our whole faith would waver. For whence should we know that He had not lied ? From revelation ? But about this very revelation there would be the same difficulty, the same doubt, whether He does not lie.

Confirmed 2°. A lie is intrinsically evil and a sin. Therefore it cannot be absolutely from God.

I say 2° : Falsehood cannot be subject to faith on the part of the believer², that is, faith cannot incline toward a false assent, nor elicit it.

It is proved. 1° Nothing is subject to a power or a habit, or even to an act, except through the formal reason of its object : just as color cannot be seen except through light, nor can a conclusion be drawn except through the middle term of a demonstration. But the formal reason of faith is the first truth in speaking, which, as we were just saying, cannot reveal falsehood. Therefore.

2° An intellectual virtue cannot be the principle of a false assent, just as the virtue of the rational appetite cannot be the principle of a vicious act. But faith is an intellectual virtue. Therefore. — Proof of the major. Every virtue is a good disposition and perfection of a power in relation to its object. But the object of the intellect is the true. Therefore.

Against the first conclusion certain apparent lies referred to in the Scriptures could be objected. But you have them abundantly solved in our larger Work, tract. de Relig., dissert. IX, a. 2, where mental restriction is treated, of which we shall also speak below at the place cited.