And I will quote here a little of the principles that are required for our topic from the Thirteen Principles that the Rambam wrote in the Commentary on the Mishnah in the chapter [entitled] Chelek and which is brought by the Rabbi of Shvilei Emunah. And [these are his words]: The first principle is to believe that the Creator, may He be blessed, exists with complete existence, that He is the cause of the existence of all that exists and that the source of their existence is from Him.
And if we could imagine the cessation of His existence, the existence of all that exists would be nullified. But if we were to imagine the nullification of totally everything that exists besides Him, His existence would not be nullified on their account, nor would it [even] be lacking. For the Creator, may He be blessed, is abounding; as He does not need the existence of anything else. And everything besides Him of intelligences, planetary bodies and that which is in them all need Him, may He be blessed, for their existence. [But] He does not need them.
And this principle is hinted in the phrase, "I am the Lord, your God." And the second principle is His unity, may He be blessed. And that is to know that the Cause of everything is one. Not one that is a composite specie, like the human species which includes many individual people.
Nor is He like one that is an individual within the specie, as you would say about an individual person, yet you would include 248 limbs in him. Nor is He one that is constructed, which can be divided into many ones. Nor is He like a simple (unconstructed) body which is unique, but can [still] be divided without end. For each one of these is called, "one," by way of borrowing [from the true meaning of the word].
For these things that are grouped together by that word are [only actually unified by being] the same regarding [that] one matter, but they are not truly one. Indeed, the true One is the unity of the Creator, may He be blessed, about whom there is no oneness like Him in any fashion. And that is His, may He be blessed, saying, "Hear O Israel, the Lord is our God, the Lord is One" (Deuteronomy 6:4). To here are his [words].