The oneness of God is one of His foremost attributes. All the heavenly religions, in their original and undistorted teachings, have summoned mankind to a pure affirmation of God's unity, untainted by the ascription of partners to Him. Such ascription of partners, in all its forms and dimensions, is the most harmful error to which man is liable. It has occurred throughout history as a result of ignorance, unawareness, and turning away from the guidance of reason and the teaching of the Prophets.
If men believed in God according to correct thought, the proofs of reason and the guidance of the Prophets, it would be impossible for them to accept any contingent phenomenon or created thing in His place, and to imagine that any other being might be His partner or equal in commanding and controlling the destinies of the world, or even have some share in administering the order of the universe.
If numerous gods ruled over the world and each of these gods acted and gave commands in accordance with his own will, the order of the universe would dissolve into anarchy.
The Quran says: "If there were numerous gods other than the one true God" the order of the heavens and the earth would collapse. So exalted be the Lord of the Throne above what they say concerning Him."(21:22)
If we say that God is one, it is because He is not a body. A body is a compound of a series of different elements, the union of which causes it to come into being. Compounding, division and generation are all attributes of contingent beings and bodies; we, therefore, negate them in the case of God and assert that whatever has come into existence, as a result of compounding and generation, neither is God nor resembles Him.
It is feasible to conceive of plurality within a given category once we speak of limitations such as quantity, quality, and time. God, however, is not limited by any of these, and it is, therefore, impossible to conceive of Him having any like or congener.
If we try to imagine the essence of water, without any limiting attribute, and repeat this exercise several times, nothing will be added to our original conception. Because in the beginning we conceived of water in an absolute sense, not limited by any condition, quantity or quality, it is impossible that in our subsequent attempts to conceive of it, a new hypothesis should occur to us.
But when we add to the essence of water certain limiting at tributes which are extrinsic to it, different forms and instances of water will appear and with them, plurality. Examples of this would be rainwater, springwater, river water, sea water, all of these observed at different times and in different places, here and there. If we eliminate all these limiting attributes and look again at the fundamental essence of water, we will see that it is exempt from all duality and is a single essence.
We must be aware that any being which can be contained in a certain place necessarily has need of that place, and any being that be contained in a certain time owes its very existence to the defining conditions of that time: its existence will be realized only within the specific temporal framework where those conditions obtain.
So, when we come to know a being that is present at all times and in all places and who possesses the highest conceivable degree of perfection, and other than whom nothing is perfect or absolute and free from defect, we must recognize that to impute duality to such a lofty reality is to make it finite and limited.
Indeed, God is not one in a numerical sense so that we might imagine Him to be the first member of a category that is followed by a second. His oneness is such that if we imagine a second to exist with Him, that second must be identical with the first.
Since the multiplicity of things derives from the limiting circumstances that differentiate them from each other, it would be totally irrational to posit a second for a being that is free of all limits and bounds. The existence of a second would mean that the first had limits and bounds, and if limits and bounds are excluded, we cannot possibly have two beings; our conception of the second will simply be a repetition of the first.
The doctrine of divine unity means that if we consider God alone, to the exclusion of all phenomenal being, His sacred essence is completely affirmed. Likewise, if we regard His being together with phenomenal being, again His existence will be completely affirmed. But if, on the contrary, we look at contingent phenomena to the exclusion of God, they cannot in any way be said to be existent, because their existence is dependent on the Creator for its origination and perpetuation.
So, whenever we ascribe some limit and condition to God, it means that God will cease to exist whenever that limit and condition cease to exist. However, God's existence is not subject to condition and plurality, and reason cannot, therefore, posit a second member of His category.
Let us give an illustration. Suppose that the world is infinite it has no bounds and in whatever direction we travel, we never come to its end. With such a concept of the world of bodies, all of its dimensions being infinite, can we imagine another world to exist in addition to it, whether finite or infinite? Certainly we cannot, because the concept of an infinite world of bodies necessarily excludes the existence of another such world. If we try to conceive of another such world, it will be either identical with the first world or a segment of it.
So, considering that the divine essence is absolute being, to posit the existence of a second being resembling Him is exactly the same as imagining a second world of bodies to co-exist with an infinite world of bodies. In other words, it is impossible.
It is, thus, clear that the meaning of God's being One is not that He is not two; it is that a second is inconceivable and that the exclusive possession of divinity is necessitated by His essence. He becomes distinct from other than Himself, not by means of any limit but by means of His essence itself which can clearly be distinguished from all else. All other beings, by contrast, attain their distinctiveness not from their essence but rather from God.