Second of the Thirteen Principles of the Jewish Faith laid down by the Rambam in the twelfth century, and the second of the five principles relating to the Existence and Nature of G-d:
“I believe with complete faith that the Creator, Blessed is His Name, is unique, and there is no uniqueness like His in any way, and that He alone is our G-d, Who was, Who is, and Who always will be.”
Nor is His Unity comparable to any other Unity. For example, the quality of the number 1, which might have seemed a reasonable candidate for similarity, is seen upon further analysis to be radically different in that it is divisible into fractions, whereas HaShem is not divisible in any way.
The idea that there is one G-d is called monotheism. The first human being to conceive of that notion was Avraham “Avinu.” Avraham was born into an idol-worshipping family; in fact, according to the Midrash, Avraham’s father was a purveyor of idols. The precocious son soon became dissatisfied with the ancient pagan gods. He realized that there must be only one G-d. His first hypothesis was that the sun was the god of the universe. But when the sun set and seemed to disappear, he realized that was impossible. He next considered the moon, with similar results. He looked heavenward and called out, “Who is the owner of the palace?” And HaShem, who had been carefully watching his progress, called to him, “I am the owner of the palace.”
Moshe “Rabbeinu” inscribed this doctrine of Faith in the Torah in the verse, “Hear O Israel, HaShem is our G-d; HaShem is One.” (Devarim 6:4)
The poetic rendition of this “Ani Maamin” in “Yigdal” is:
“He is One – and there is no Unity like His Oneness; Inscrutable and Infinite is His Oneness.”