The truth of the oneness of God ranks high among the elements of knowledge that God wanted us to be able to acquire with ease and facility. “We believe in one God” is the statement of a confession of faith based on supernatural revelation. Both the Old and New Testaments contain explicit references to the existence of one true God.
Important also is the fact that the truth of the oneness of God is a truth accessible to reason. Although grasping and believing the revealed statements is certainly easier than reasoning to the conclusion that God is one, several arguments from reason have been advanced to show that there cannot be more than one God. I shall first present a brief summary of four of these arguments.
The argument of the unmoved mover
The proof for the existence of God based on the observation of motion is well known. It demonstrates more specifically the existence of a first unmoved mover. Now, this first unmoved mover can only be one.
The continuous and regular movement we observe in the universe cannot be the effect of many movers. The reason is this. If the observation of movement is to be explained by the activity of many movers, then the many movers either cause the movement simultaneously or non-simultaneously. If the many movers do not simultaneously cause the movement, then some of them at times move and at times don’t move. But a mover which does not always move can only be the cause of irregular movement. So, many movers cannot be the ultimate cause of movement in a non-simultaneous way.
On the other hand, if the many movers are needed to simultaneously cause the movement, then none of them would be the first mover because by itself none of them has the power to cause the continuous and regular movement of the universe. And this, of course, will not satisfy the search for the ultimate cause of movement either. Therefore the first mover cannot be more than one. And this first mover is the unmoved mover which Saint Thomas Aquinas identified with God. There can be no more than one God.
The argument of the being who is pure act
The proof for the existence of God based on the observation of motion also leads to knowledge of the existence of a being who is pure act. Now, just as the unmoved mover cannot be more than one, so a being who is pure act cannot be more than one either.
For if there were to be more than one being who is pure act, then there would have to be some way of distinguishing one from another. This can be done only if one has some perfection or privation that the other lacks. But a being who is pure act cannot have and does not have any imperfection or privation because imperfections and privations are ways of failing to be pure act. Thus, there would be no way of distinguishing one being who is a pure act from another who is pure act also. In consequence, a being who is pure act cannot but be one, and this is God. There can be no more than one God.
The argument of the supreme good
There is also proof for the existence of God based on the observation of degrees of perfection in the things of nature. This observation leads to the existence of a maximum. The basic premise of the proof is the observed fact that there exists in nature a gradation of goodness, truth, beauty, and such like perfections which are not limited in their signification to material beings. More specifically, in the case of goodness, the proof leads to the existence of a supreme good.
Now, it is impossible that there be two supreme goods because if one supreme good didn’t have what the other has, then none of the two would be supreme and perfect. And if none of the two supreme goods lack anything from what the other has, then they would not be different but one and the same.
The reasoning is equally valid if there were to be several supreme goods. To account for multiplicity, each supreme good needs to have a distinguishing feature which would make them distinct from one another. But a distinguishing feature would preclude their being supreme and perfect. Therefore the supreme good cannot but be one, and this is God.
The argument of the individuation of God’s essence
Nothing that belongs to an individual as individual can belong to another because the individuality of a thing does not belong to any other thing. Now, it can be demonstrated that God’s essence is being itself. God’s essence is unparticipated being. This means that the fullness of being belong to God as an individual.
If there is more than one God, then the distinct Gods would have to share their essence while each having its own individual being. But it is impossible for an individual whose essence is unparticipated being to share what belongs to it individually, for then this individual would no longer be unparticipated being. Therefore the fullness of being cannot but belong to one individual only, and this is God. There can be no more than one God.
The oneness of God in Sacred Scripture
The inspired writers explicitly state that God is one. “Hear, O Israel: The Lord your God, the Lord is one” (Mk 12:29). “The Lord is one” (Deut 6:4). “There is no God but one” (1 Cor 8:4). “For us there is one God” (1 Cor 8:6). “An intermediary implies more than one, but God is one” (Gal 3:20). “The Lord is God, there is no other besides him” (Deut 4:35). “You shall have no other gods besides me” (Ex 20:3). “O Lord God, there is no God besides you” (2 Sam 7:22). “And this is eternal life, that they may know you the only true God” (Jn 17:3).
Revelation and reason converge and complement each other in establishing the truth of the oneness of God.