It can be shown from these three sources that God is one.
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Secondly, this is proved from the infinity of His perfection. For it was shown above (Q.4, A.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….
Summa Theologiae I, Q. 11, A. 3
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