G-d Is Not A Mortal
Maimonides devotes most of the "Guide for the Perplexed" to the fundamental idea that G-d is Incorporeal, meaning that He assumes no physical form.
G-d is Eternal, above time. He is Infinite, beyond space. He cannot be born, and cannot die.
Saying that G-d assumes human form makes G-d small, diminishing both His Unity and His Divinity.
As the Torah says: "G-d is not a mortal" (BaMidbar 23:19).