Stretch your mind with some monotheistic philosophy:
The self-existent Being (God) is identical with its nature (of being existence itself). (This is distinct from all other beings, which merely 'have' existence) With other beings, such as man, nature is something possessed by a number of distinct individuals. That is, human nature, is something possessed by a number of individuals. Whereas, the Being that is existence can only be one, for the same reason existence itself can only be one, since there is no middle ground between existence and non-existence.Thats some good stuff.
For there to be more than one Being of this nature, there must be a distinction between the two or more. For instance, if there is no distinction between what is meant by A and what is meant by B, A and B are identical, i.e.: differing only insofar as they are called different things (A and B). Self-existence consists in unlimited perfection, and therefore, to suppose two or more beings that are self-existent, is to suppose two or more beings that are unlimited perfection.
This means there would be no distinction, (i.e.: they are exactly the same being), and so there is only one Being in the first place. The only way to get a distinction is to suppose one being to have more or less perfection, i.e.: some difference between the two self-existent Beings through which they get a distinct identity. More perfection is contradictory, as it is already unlimited. Less perfection would signify a limited perfection - something impossible for something self-existent.