Maimonides felt that G-d is transcendent, that G-d has no body and is one, though we cannot attribute any description to G-d. At best, we can say what G-d is not. Thus it is impossible to describe G-d with language. Thus, Maimonides was clear that we can only refer to G-d with silence.
"The most apt phrase concerning this subject is the dictum occurring in the Psalms, Silence is praise to Thee (Ps. 65:2), which interpreted signifies: silence with regard to You is praise." (Guide, 1:59)
In addition to praising G-d with silence, Maimonides writes that all we can say is what G-d is not:
"Know that description of G-d, may He be cherished and exalted, by means of negation is the correct description—a description that is not affected by an indulgence in facile language and does not imply any deficiency with respect to G-d in general or in any particular mode. On the other hand, if one describes him by means of affirmations, one implies, as we have made clear, that he is associated with that which is not He and implies a deficiency in Him." (Guide, 1:58)
In other words, to describe G-d diminishes His stature, if not insulting. For describing G-puts G-d into a category. If we say, for example, that G-d is good, then we are placing G-d in a category of good people. This seems to contradict what the Bible teaches, that G-d is merciful, gracious, good, etc. But the Bible focused on the uniqueness of G-d, not necessarily on His oneness. For example, some scholars feel that the words in the Shema "G-d is “one," may refer to the unique characteristics of G-d, a G-d separated (holy) from nature. For many ancient cultures held the belief that there was one creator, even the ancient Egyptians believed at one point in a monotheistic religion, for example, the Sun-disc god called Aton (represented by the sun because the Egyptians felt that there was one sun). However, the Egyptians were still an idolatrous culture and their god (or, gods) were still subject to nature. In any event, the Bible's revolution did not focus on monotheism per se but that G-d’s “oneness” is unique.
In the Guide, Maimonised used metaphysical proofs in what he called the doctrine of negative attributes (beginning of part 2 of the Guide) to prove G-d's oneness. G-d is not an impossible existence nor possible, G-d is a necessity. An ultimate source that prompts, for example, the motions of the spheres. G-d is an absolute unity and from this, we can infer His existence. G-d is not “a kind that encompasses many other single particulars,” nor “a body that is divided into many parts and extremities.”
"This G-d is one; He is neither two nor more than two; He is simply one." (MT, Hilkhot Yesodei HaTorah, 1:7)
The term “one” here denotes nothing in this world that we can refer. Which is to say, anything we can call or attribute to "oneness" in the material world is only a designation of a singularity. Any physical object can ultimately be divided into elements that make up a single object in question and can be divided further into chance motion atoms bouncing around which makes the illusion of "oneness." It is merely a linguistic device. Maimonides explains,
"If the Creator had a body, He would have a defined form for it is impossible that there should be a body that is not defined. And anything that is defined is limited in its power." (MT, Hilkhot Yesodei HaTorah, 1:7)
Since G-d is not made up of atoms, it follows that only G-d can be described as "one." A unitary. Because G-d’s unity is not dependent on material things, G-d is infinite and immaterial, incorporeal. For if G-d were many (multiplicity) it would require form which can be distinguished (though physical characteristics).
"If G-d were many, He would have a body and physicality, because items that are co-extensive with each other cannot be counted as distinct from one another except through occurrences that happen to their bodies." (MT, Hilkhot Yesodei HaTorah, 1:7)
Thus, it is impossible to apply the term “one” to anything else other than G-d. For only G-d is truly “one.” This is explicit in the opening chapter of Mishneh Torah. However, in the Guide Maimonides explicitly explains the “Doctrine of Negative Attributes” which effectively prohibits any kind of verbal description to G-d, making it is impossible to characterize G-d.
So, which one is it? Can we describe G-d as "one" and yet say it is impossible to describe G-d with any kind of language?
 For example, Maimonides criticized Christianity because what bothered him the most was the notion that there are three gods. He felt strongly that this is philosophically impossible.
 See also Mishneh Torah, Hilkhot Yesodei HaTorah, 3; see also Guide, 2:1: “it is impossible that He should be two or more, because of the impossibility of a multitude of separate things that have no physical form.”