St. Thomas Aquinas’ arguments for the ‘Oneness’ of God are several.
The first says that if there were many gods, they would be called ‘god’ either univocally or equivocally. If equivocally, then he says that “this is not relevant to our topic.” (Short and simple.)
If univocally, then they must all be the same in genus or species, but he showed that God can be neither genus nor species, and therefore multiple gods are impossible.
The second argument is fairly straightforward – It is impossible for one thing to be multiple things.
“If an essence is individuated by itself, it cannot pertain to many.“
Because God’s essence is His existence, the divine essence is individuated by itself. The following conclusion is the same as the first. God can not be more than one.
I won’t go into the third due to general laziness and a lack of sufficient cause.
While giving one of my brothers (who is about a decade younger than I) a random quiz on what he is studying in his Religion books, I was given a response to the question: Why can there be only one God?
His answer was remarkably close to that which I gave my grandmother in reply to the same question about ten years ago: “There can be only one God because everything made works together with everything else in the world.”
His argument is that because everything he has observed about the natural world acts and reacts so perfectly together, it seems unreasonable to think that there could be more than one Mind and Designer behind it all. This, from a ten year old who is quite diligent in his reading on natural science, particularly zoology. Rather than strengthening the claim that some have made that knowledge of the world and of God are incompatible, his studies are showing him more and more of the Beauty and Wisdom of his Creator.
When I answered my grandmother ten years ago, I added that if there were many gods then either they would all be equal and thus no Supreme Being.
(It was simple math. Many things equal to one another are all equal.)
My brother this morning, with a little help, got around to that reasoning as well, and also found that if there were many gods, and one of them was ‘higher’ than the rest, that he would be supreme and thus God (according to the definition of God as ‘Supreme Being’).
It seems that anyone can, without having to understand genus or species, form or essence, discover the ‘Oneness’ of Divinity, and I for one find myself relieved to know that it is not necessary to have the intellectual capability that St. Thomas had. If that were so, I for one would be quite lost.
Makes me wonder why most of the world before Christ and for a long time afterwards was pagan; full of people who worshiped multiple gods. Perhaps because there are innumerable spiritual beings who are not in compliance with the Will of God. Demons can (and do) mimic the Truth, appealing to fear and concupiscence, showing the good as something rightfully ones own rather than a gift, displaying the false advertisement of Divinity to men as though we could become gods.
Hopefully more thoughts (I mean actual thoughts) on that later . . .