Regarding these 13 principles I prefaced some remarks on the parent page here
The second of the Rambam’s principles is about the Unity of G-d. The Rambam writes that this entity which is the cause of all that is, is One. But not the kind of “one” that we know of. Any one thing that we are familiar with in the world, like one person, can be divided into parts of that one. Even an amoeba. G-d is a simple unity, one that cannot be divided into parts.
The Rambam writes that this principle is included in the first verse of the Shema that we say daily, Shema Yisroel, Hashem Elokenu, Hashem Echad. Not only One as opposed to Two, but even One as opposed to something which is comprised of parts.
I wrote elsewhere, and I repeat it here because it is relevant, that one should be warned: the essence of G-d is beyond our comprehension, and the more you think about it, the more you try to envision what He is, the more you are in danger of going off the track. It is human nature to try to conjure up an understanding which is based on concepts which are part of the creation. This is already off base. Leave it. G-d is One, and that’s all you have to know.
