Today’s catechism section deals with the “oneness” of God. A study of the attributes of God wouldn’t be complete without the words of St. Thomas Aquinas, who provides today’s supporting material.

Article 1

"I BELIEVE IN GOD THE FATHER ALMIGHTY, CREATOR OF HEAVEN AND EARTH"

Paragraph 1. I BELIEVE IN GOD

199 "I believe in God": this first affirmation of the Apostles' Creed is also the most fundamental. The whole Creed speaks of God, and when it also speaks of man and of the world it does so in relation to God. The other articles of the Creed all depend on the first, just as the remaining Commandments make the first explicit. The other articles help us to know God better as he revealed himself progressively to men. "The faithful first profess their belief in God."2

I. "I BELIEVE IN ONE GOD"

200 These are the words with which the Niceno-Constantinopolitan Creed begins. The confession of God's oneness, which has its roots in the divine revelation of the Old Covenant, is inseparable from the profession of God's existence and is equally fundamental. God is unique; there is only one God: "The Christian faith confesses that God is one in nature, substance and essence."3

201 To Israel, his chosen, God revealed himself as the only One: "Hear, O Israel: the LORD our God is one LORD; and you shall love the LORD your God with all your heart, and with all your soul, and with all your might."4 Through the prophets, God calls Israel and all nations to turn to him, the one and only God: "Turn to me and be saved, all the ends of the earth! For I am God, and there is no other... To me every knee shall bow, every tongue shall swear. 'Only in the LORD, it shall be said of me, are righteousness and strength.'"5

202 Jesus himself affirms that God is "the one Lord" whom you must love "with all your heart, and with all your soul, and with all your mind, and with all your strength".6 At the same time Jesus gives us to understand that he himself is "the Lord".7 To confess that Jesus is Lord is distinctive of Christian faith. This is not contrary to belief in the One God. Nor does believing in the Holy Spirit as "Lord and giver of life" introduce any division into the One God:

We firmly believe and confess without reservation that there is only one true God, eternal infinite (immensus) and unchangeable, incomprehensible, almighty and ineffable, the Father and the Son and the Holy Spirit; three persons indeed, but one essence, substance or nature entirely simple.

IN BRIEF

228 "Hear, O Israel, the LORD our God is one LORD..." (⇒ Dt 6:4; ⇒ Mk 12:29). "The supreme being must be unique, without equal. . . If God is not one, he is not God" (Tertullian, Adv. Marc., 1, 3, 5: PL 2, 274).

The Angelic Doctor, Thomas Aquinas explains the “oneness” of God in his Summa Theologica (1, 11):

Article 3. Whether God is one?

Objection 1. It seems that God is not one. For it is written "For there be many gods and many lords" (1 Corinthians 8:5).

Objection 2. Further, "One," as the principle of number, cannot be predicated of God, since quantity is not predicated of God; likewise, neither can "one" which is convertible with "being" be predicated of God, because it imports privation, and every privation is an imperfection, which cannot apply to God. Therefore God is not one.

On the contrary, It is written "Hear, O Israel, the Lord our God is one Lord" (Deuteronomy 6:4).

I answer that, It can be shown from these three sources that God is one.

First from His simplicity. For it is manifest that the reason why any singular thing is "this particular thing" is because it cannot be communicated to many: since that whereby Socrates is a man, can be communicated to many; whereas, what makes him this particular man, is only communicable to one. Therefore, if Socrates were a man by what makes him to be this particular man, as there cannot be many Socrates, so there could not in that way be many men. Now this belongs to God alone; for God Himself is His own nature, as was shown above (Question 3, Article 3). Therefore, in the very same way God is God, and He is this God. Impossible is it therefore that many Gods should exist.

Secondly, this is proved from the infinity of His perfection. For it was shown above (Question 4, Article 2) that God comprehends in Himself the whole perfection of being. If then many gods existed, they would necessarily differ from each other. Something therefore would belong to one which did not belong to another. And if this were a privation, one of them would not be absolutely perfect; but if a perfection, one of them would be without it. So it is impossible for many gods to exist. Hence also the ancient philosophers, constrained as it were by truth, when they asserted an infinite principle, asserted likewise that there was only one such principle.

Thirdly, this is shown from the unity of the world. For all things that exist are seen to be ordered to each other since some serve others. But things that are diverse do not harmonize in the same order, unless they are ordered thereto by one. For many are reduced into one order by one better than by many: because one is the "per se" cause of one, and many are only the accidental cause of one, inasmuch as they are in some way one. Since therefore what is first is most perfect, and is so "per se" and not accidentally, it must be that the first which reduces all into one order should be only one. And this one is God.

Reply to Objection 1. Gods are called many by the error of some who worshipped many deities, thinking as they did that the planets and other stars were gods, and also the separate parts of the world. Hence the Apostle adds: "Our God is one," etc.

Reply to Objection 2. "One" which is the principle of number is not predicated of God, but only of material things. For "one" the principle of number belongs to the "genus" of mathematics, which are material in being, and abstracted from matter only in idea. But "one" which is convertible with being is a metaphysical entity and does not depend on matter in its being. And although in God there is no privation, still, according to the mode of our apprehension, He is known to us by way only of privation and remotion. Thus there is no reason why a certain kind of privation should not be predicated of God; for instance, that He is incorporeal and infinite; and in the same way it is said of God that He is one.

Article 4. Whether God is supremely one?

Objection 1. It seems that God is not supremely "one." For "one" is so called from the privation of division. But privation cannot be greater or less. Therefore God is not more "one" than other things which are called "one."

Objection 2. Further, nothing seems to be more indivisible than what is actually and potentially indivisible; such as a point and unity. But a thing is said to be more "one" according as it is indivisible. Therefore God is not more "one" than unity is "one" and a point is "one."

Objection 3. Further, what is essentially good is supremely good. Therefore what is essentially "one" is supremely "one." But every being is essentially "one," as the Philosopher says (Metaph. iv). Therefore every being is supremely "one"; and therefore God is not "one" more than any other being is "one."

On the contrary, Bernard says (De Consid. v): "Among all things called one, the unity of the Divine Trinity holds the first place."

I answer that, Since "one" is an undivided being, if anything is supremely "one" it must be supremely being, and supremely undivided. Now both of these belong to God. For He is supremely being, inasmuch as His being is not determined by any nature to which it is adjoined; since He is being itself, subsistent, absolutely undetermined. But He is supremely undivided inasmuch as He is divided neither actually nor potentially, by any mode of division; since He is altogether simple, as was shown above (Question 3, Article 7). Hence it is manifest that God is "one" in the supreme degree.

Reply to Objection 1. Although privation considered in itself is not susceptive of more or less, still according as its opposite is subject to more or less, privation also can be considered itself in the light of more and less. Therefore according as a thing is more divided, or is divisible, either less or not at all, in the degree it is called more, or less, or supremely, "one."

Reply to Objection 2. A point and unity which is the principle of number, are not supremely being, inasmuch as they have being only in some subject. Hence neither of them can be supremely "one." For as a subject cannot be supremely "one," because of the difference within it of accident and subject, so neither can an accident.

Reply to Objection 3. Although every being is "one" by its substance, still every such substance is not equally the cause of unity; for the substance of some things is compound and of others simple.

Footnotes:

2 Roman Catechism I, 2, 2.
3 Roman Catechism I, 2, 2.
4 Dt 6:45.
5 Is 45:22-24; cf. ⇒ Phil 2:10-11.
6 ⇒ Mk 12:29-30
7 Cf. ⇒ Mk 12:35-37.