God is ONE, this is the core creed of Monotheism. This oneness of God as an aspect of His absolute nature, must be understood properly in order to worship HIM.

Maimonides[1] had laid out a principle to understand the Oneness of God based on the Hebrew Bible. The Oneness which is absolute and unique, a Oneness that knows no parallel:

The Oneness of God, (may He be blessed), which is to say that we believe that God, who is the cause of everything, is ONE.

and not like one of a pair
and not like one of a group
and not like one person that can be divided into many units
and not like a simple body which is numerically one [but] can be infinitely divided.

rather God, may He be blessed, is ONE in a oneness that has no unity like it.
and this is the second principle, [and] it is indicated by that which is stated:

“Hear O Israel, God is our Lord, God is One.”

Maimonides ruled out any possibility that the Hebrew adjective echad אחד actually means a “compound unity” like those trinitarian expositors who love to argue that the shema foreshadows a trinity. The oneness of God is independent of anything -thus rejecting the subtle influences of polytheism which could exist even in a monotheistic system – that is to say that the Almighty God could ever be involved in the act of begetting and coming down as man[2].

It is striking that the most important Hebrew creed, the Shema, remains preserved in the Qur’ān, in Sūrat al-ikhlāṣ:

1. Say (O Muhammad): “He is Allah, (the) One (Ahad).

2. “Allah-us-Samad (The Self-Sufficient Master, Whom all creatures need, He neither eats nor drinks).

3. “He begets not, nor was He begotten;

4. “And there is none co-equal or comparable unto Him.”

It employs its Arabic homophonic noun Ahad أَحَد, instead of the more pertinent adjective واحد Wāhid [3].  Muslim scholars have long established that grammatically the word ahad conveys an uncountable oneness. It is not one in a series like wāhid. It is a singular, unique entity. It is referring specifically to God’s essence, which is absolutely singular and utterly unique in His attributes. No one is like Him in any way[4].

Thus the Qur’an clearly affirms and, at the same time, corrects two powerful earlier creeds, the Shema and the Nicene, and set itself as the ultimate universal monotheistic creed [5].

Notes

1. Maimonides is also known an as Rabbi Moshe ben Maimon (Ar: موسى بن ميمون‎‎ Mūsā bin Maymūn) or Rambam(1137 – 1204 CE). He was considered the greatest intellectual and spiritual figure of post-Talmudic Judaism. He wrote authoritative works of philosophy, Halacha, commentary, and responsa. His works were all foundational in their field. He was the first to produce a comprehensive commentary on the entire Mishnah. All of his works were written in Judeo-Arabic except for Mishnah Torah, which was written in Hebrew.  He and his descendants served as Negidim (leaders) of Egyptian Jews for five generations.
2. Nicene creed: “And in one Lord Jesus Christ, the Son of God, begotten of the Father [the only-begotten; that is, of the essence of the Father, God of God,] Light of Light, very God of very God, begotten, not made, being of one substance with the Father”; …”Who for us men, and for our salvation, came down and was incarnate and was made man“.
3. Allah’s name al-Wāhid (the One) appears in twenty-two verses of the Qur’ān. The name al-Ahad appears only once, in the 112th chapter of the Qur’an: al-Ikhlās.
4. “There is nothing like unto Him, and He is the Seeing, the Hearing.” [Sūrah al-Shūrā: 11]
5. Angelika Neuwirth, Two Faces of the Qur’ān: Qur’ān and Muṣḥaf, Oral Tradition, 25/1 (2010): p. 151-153.
   Verse 3—”He did not beget nor is he begotten”; lam yalid wa-lam yūlad—is a reverse echo of the Nicene creed; it rejects the emphatic affirmation of Christ’s sonship—begotten, not made; gennêthenta, ou poiêthenta—by a no less emphatic double negation. A negative theology is established through the inversion of a locally familiar religious text. This negative theology is summed up in verse 4—“And there is none like Him”; wa-lam yakun lahu kufuwan aḥad. The verse that introduces a Qur’ānic hapax legomenon, kufuwun, “equal in rank,” to render the core concept of homoousios, not only inverts the Nicene formula of Christ’s being of one substance with God—homoousios to patri—but also forbids thinking of any being as equal in substance with God, let alone a son.