One is convertible with Being, and adds nothing to it but the negation of division. Everything is either simple or compound; what is simple is undivided and indivisible; what is compound has no existence whilst its parts are divided, but only when they make and compose the compound; hence the existence of anything consists in the absence of division, and it follows that a thing maintains its being accordingly as it preserves its unity.
One is opposed to many, but in different ways, for one, which is the beginning of numbers, is opposed to multitude as the measure is to the thing measured, because One represents the first measure, and number is multitude measured by one, while One, as it is interchangeable with Being, is opposed to multitude by way of privation of multitude, as the undivided is to the divided.
God is One, for that which causes a thing to be singular cannot be communicated to many; and this belongs to God, for God Himself is His own Nature; hence "God" and "this God" are the same. Wherefore there cannot be many gods, which is also evident from the fact that God comprehends in Himself the whole perfections of existence. If there were several gods, something would belong to one and not to another, and so he in whom was privation of anything would not be simply perfect. The same is likewise proved by. the unity of the world, for one is the cause of one, and things are better arranged by one than by many, and are brought into a uniform order. Thus God is pre-eminently One, because He is pre-eminently Being, inasmuch as He has not any existence restricted to any other nature, but He is Himself His own Existing Essence, entirely indeterminate and absolutely undivided in act and potentiality, for He is in every way Simple. Hence God is pre-eminently One.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni