(CG 1/040). That God is the Good of all Good (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

THERE cannot possibly be two sovereign goods. But God is the sovereign good. Therefore there is but one God.

2. God is all-perfect, wanting in no perfection. If then there are several gods, there must be several thus perfect beings. But that is impossible: for if to none of them is wanting any perfection, nor is there any admixture of imperfection in any, there will be nothing to distinguish them one from another.

7. If there are two beings, each necessarily existent, they must agree in point of necessary existence. Therefore they must be distinguished by some addition made to one only or to both of them; and thus either one or both must be composite. But no composite being exists necessarily of itself, as has been shown above (Chap. XVIII). Therefore there cannot be several necessary beings, nor several gods.

9. If there are two gods, this name 'God' is predicated of each either in the same sense or in different senses. If in different senses, that does not touch the present question: for there is nothing to prevent anything from being called by any name in a sense different from that in which the name is ordinarily borne, if common parlance so allows. But if the predication is in the same sense, there must be in both a common nature, logically considered. Either then this nature has one existence in both, or it has two different existences. If it has one existence, they will be not two but one being: for there is not one existence of two beings that are substantially distinct. But if the nature has a different existence in each possessor, neither of the possessors will be his own essence, or his own existence, as is proper to God (Chap. XXII): therefore neither of them is that which we understand by the name of God.

12. If there are many gods, the nature of godhead cannot be numerically one in each. There must be therefore something to distinguish the divine nature in this and that god: but that is impossible, since the divine nature does not admit of addition or difference, whether in the way of points essential or of points accidental (Chap. XXIII, XXIV).

13. Abstract being is one only: thus whiteness, if there were any whiteness in the abstract, would be one only. But God is abstract being itself, seeing that He is His own being (Chap. XXII). Therefore there can be only one God.

This declaration of the divine unity we can also gather from Holy Writ. For it is said: Hear, O Israel, the Lord your God is one Lord (Deut. vi, 4) And, One Lord, one faith (Eph. iv, 5).

By this truth the Gentiles are set aside in their assertion of a multitude of gods. Yet it must be allowed that many of them proclaimed the existence of one supreme God, by whom all the other beings to whom they gave the name of gods had been created. They awarded the name of godhead to all everlasting substances, chiefly on the score of their wisdom and felicity and their government of the world. And this fashion of speech is found even in Holy Scripture, where the holy angels, or even men bearing the office of judges, are called gods: There is none like you among gods, O Lord (Ps. lxxxv, 8); and, I have said, Ye are gods (Ps. lxxxi, 6). Hence the Manicheans seem to be in greater opposition to this truth in their maintenance of two first principles, the one not the cause of the other.