God is one and has no partner
The unity of God could be established in manifold manners but in this lesson we will present two of them.
The first argument
The solidarity and coordination of the universe by itself is a strong evidence of God’s unity. Imagine that there is a five-hundred-page book before you which you have not yet read. As yet you don’t know whether it is the work of the mind and pen of one author or each section of the book has been written by one separate writer. To establish this, you will have to read the entire book. If you realize that the spirit of the contents, the style of the arguments and organization of the sentences are unified and harmonious, you will definitely conclude that the whole book is written by one writer, because if it were co-authored by two or more authors, you would have come up with a difference. No matter how subtle, hidden, and insignificant that difference is, so that it won’t be discovered by laymen, it won’t evade the scrutiny of experts who will figure out that the book is not the work of one person.
Given this introduction, we begin studying the universe which has been called “God’s book” by some scientists. We will consult the experts of every science pertinent to the world to see whether all the pages of this magnificent book of creation are unified and harmonious or not. If it so, it will be the strongest testimony to the unity of its Creator. In fact, when we pose this question to the scientists of these disciplines, they reaffirm the unified order that is prevalent throughout the world and they contend that this world is founded upon a unified plan and its rules are so encompassing, comprehensive and harmonious that an experiment with a single creature would reveal a general rule that is valid everywhere. All scientists, including biologists, physicists, and astronomers attest to the existence of this harmony and coordination.
Could harmony and similarity be more obvious than this that the solar system and other gigantic systems are subject to the same rule to which the tiny atom is subject. As we all know, the revolution of the big planets around their orbits results from two forces, viz. “gravity” and “centrifuge.” Atoms, hundreds of millions of which don’t take up more than a millimeter, also have these two forces of “gravity” and “centrifuge.” As a result of these forces, “electrons” revolve around “protons.”
If you look at the world of plants, animals and humans, you will come across the same harmony. The general rule of reproduction among plant, animals and humans is so similar to one another that it has led some to conclude that “All species, in principle, spring from one specimen.” Could not you figure out that excerpts from different speeches extracted from a number of recordings have been delivered by one person? You could establish the fact that by means of investigating the coherence, cohesion, the similarity of style and structures.
The world is like a book composed of many pages, whose every page and line has been examined and studied by thousands of scientists. All unanimously concur that the entire length of the book is subject to series of general rules. For instance, the rule of causality is valid everywhere or the rule of gravity could be applied to the entire world of substance. All atoms have a center and the rule of light is the same everywhere.
Does not the harmony and coordination observed in the book of creation attest to the unity of its Creator? Do not you think it likely that if the world had two or more creators, it would result in diversity and discordance in the management and maintenance of the world?
Hishām, one of the youngest and most outstanding disciples of Imām aṣ-Ṣādiq (‘a) asked him, “What is the reason for the unity of God?”
The revered Imām summarized the above-mentioned argument in a concise sentence, “The continuity and coordination of the world (that are inextricably linked to one another) and the integrity and perfection of the creatures testify to the unity of the Creator.”
The second reason for the unity of God
The second reason for the unity of God is the fact that all the prophets sent to guide people who proved their authenticity with miracles and innumerable signs, encouraged people to worship the Only God and practice monotheism without any exception. It goes without saying that if there were more than one god, the prophets would not invite people to practice monotheism but would direct people to the gods by whom they were appointed. And it is quite baseless to argue that there might be gods who have not sent any prophets to guide their creatures. How is it probable that God could have been so indifferent to his most superior creature (man) and neglect to send any leader or guide to lead him toward prosperity.
The Commander of the Faithful in a letter to Imām Ḥasan (‘a) has raised the same point. “O dear son! Know that if there were any god except the Only God, it must have sent prophets. And the fact that all the prophets invite us to worship the Only God, strongly testifies to the fact that there is no god except the Only God. Besides, if God had any partner, the traces of his divine dominion and rule would be evident, when no such vestige could be traced, we are reassured that God has no partner.”
 Al-Tawḥīd by al-Ṣadūq, p.250.
 Nahj al-Balāghah, vol. 3, Egypt, p. 49.