Theologians usually use reason and deduction to prove the existence, unity and oneness of God. They use a teleological argument for the existence of God as a creator based on perceived evidence of order, purpose, design or direction or some combination of them in nature. Teleology is the supposition that there is a purpose or directive principle in the works and processes of nature.^[28]

Another argument that is used frequently by theologians is reductio ad absurdum, which they use instead of positive arguments as a more efficient way to reject their opponent's ideas.^[29]

Against the polytheism of pre-Islamic Arabia, the Quran argues that the knowledge of God as the creator of everything rules out the possibility of lesser gods since these beings must be themselves created. For the Quran, God is an immanent and transcendent deity who actively creates, maintains and destroys the universe. The reality of God as the ultimate cause of things is the belief that God is veiled from human understanding because of the secondary causes and contingent realities of things in the world.^[11] Thus, the belief in the oneness of God is equated in the Quran with the "belief in the unseen" (2:3).^[11] The Quran summarises its task in making the "unseen" become, to a greater or lesser degree, "seen" so that belief in the existence of God becomes a master truth, rather than an unreasonable belief. The Quran states that God's signals are so near and yet so far, demanding that its students listen to what it has to say with humility (50:33, 50:37). The Quran draws attention to certain observable facts to present them as "reminders" of God, instead of providing lengthy "theological" proofs for the existence and unity of God.^[30]

Ash'ari theologians rejected cause and effect in essence but accepted it as something that facilitates humankind's investigation and comprehension of natural processes. The medieval scholars argued that nature was composed of uniform atoms that were "recreated" at every instant by God. The laws of nature were only the customary sequence of apparent causes (customs of God), the ultimate cause of each accident being God himself.^[31][32] Other forms of the argument also appear in Avicenna's other works, and the argument became known as the Proof of the Truthful.

Avicenna initiated a full-fledged inquiry into the question of being in which he distinguished between essence (Mahiat) and existence (Wujud). He argued that the fact of existence may not be inferred from or accounted for by the essence of existing things and that form and matter by themselves cannot interact and originate the movement of the universe or the progressive actualization of existing things. Existence must, therefore, be caused by an agent-cause that necessitates, imparts, gives, or adds existence to an essence.

An ontological argument for the existence of God was first proposed by Avicenna (965–1037) in the Metaphysics section of The Book of Healing.^[33][34] Other forms of the argument also appear in Avicenna's other works, and the argument became known as the Proof of the Truthful. Avicenna initiated a full-fledged inquiry into the question of being, in which he distinguished between essence (Mahiat) and existence (Wujud). He argued that the fact of existence can not be inferred from or accounted for by the essence of existing things and that form and matter by themselves cannot interact and originate the movement of the universe or the progressive actualization of existing things. Existence must, therefore, be caused by an agent-cause that necessitates, imparts, gives or adds existence to an essence. To do so, the cause must be an existing thing and co-exist with its effect.^[35]

That was the first attempt at using the method of a priori proof, which uses intuition and reason alone. Avicenna's proof of God's existence is unique in that it can be classified as both a cosmological argument and an ontological argument. "It is ontological insofar as 'necessary existence' in intellect is the first basis for arguing for a Necessary Existent". The proof is also "cosmological insofar as most of it is taken up with arguing that contingent existence cannot stand alone and must end up in a Necessary Existent".^[36] Another argument Avicenna presented for God's existence was the problem of the mind–body dichotomy.^[37]

According to Avicenna, the universe consists of a chain of actual beings, each giving existence to the one below it and responsible for the existence of the rest of the chain below. Because an actual infinite is deemed impossible by Avicenna, the chain as a whole must terminate in a being that is wholly simple and one, whose essence is its very existence and therefore is self-sufficient and does not need something else to give it existence. Because its existence is not contingent on or necessitated by something else but is necessary and eternal in itself, it satisfies the condition of being the necessitating cause of the entire chain that constitutes the eternal world of contingent existing things.^[35] Thus, his ontological system rests on the conception of God as the Wajib al-Wujud (necessary existent). There is a gradual multiplication of beings through a timeless emanation from God as a result of his self-knowledge.^[38][39]

The Quran argues that there can be no multiple sources of divine sovereignty since "behold, each god would have taken away what [each] had created, And some would have Lorded it over others!"^[7] The Quran argues that the stability and order prevailing throughout the universe shows that it was created and is being administered by only one God (28:70-72).^[6][40]

The Quran in verse 21:22 states, "Had there been within them [i.e., the heavens and earth] gods besides Allāh, they both would have been ruined". Later Muslim theologians elaborated on the verse by saying that the existence of at least two gods would inevitably arise between them, at one time or another, a conflict of wills. Since two contrary wills could not possibly be realized at the same time, one of them must admit himself powerless in that particular instance. On the other hand, a powerless being can not by definition be a god. Therefore, the possibility of having more than one god is ruled out.^[6][40] For if a god is powerful above another, that asserts a difference in the particular attributes that are confined to the essence of godhood, which implies the lesser god must lack in certain necessary attributes, which make the deity as anthropomorphic and snatches the title of god away from that entity.

The Quran argues that human beings have an instinctive distaste for polytheism. At times of crisis, for example, even the idolaters forget the false deities and call upon the one true God for help. As soon as they are relieved from the danger, however, they start associating other beings with God. "If they happen to be aboard a ship 'caught in a storm', they cry out to Allah ˹alone˺ in sincere devotion. But as soon as He delivers them 'safely' to shore, they associate 'others with Him once again'." (29:65).^[40

According to the Quran, one consequence of properly conceived relationship between God and man as the served and servant is the proper relationship among humans. To achieve the former, the Quran consistently "reminds" mankind of two points: God is one; everything except God (including the entirety of nature) is contingent upon God. With all His might and glory, God is essentially the all-merciful God.^[74]

According to the Quran, God is the progenitor of all things, both good and evil.^[75] As is written in the Quran, all of humanity is created at the will of Allah, both the good and the evil, and their natures have been predisposed as such since the beginning of creation.^[76][77]

According to the Quran, Satan deviated from the unification of God in the story of creation of man by permitting his own hierarchical value system to supersede God's will: God asked the angels to bow to Adam, who he had created from clay. Satan refused and said, "I am better than him; you created me from fire and created him from clay". The medieval Muslim scholar Al-Ghazali, pointing out that the only legitimate "preference principle" in the sight of God is piety, wrote, "Every time a rich man believes that he is better than a poor one, or a white man believes that he is better than a black one, then he is being arrogant. He is adopting the same hierarchical principles adopted by Iblis [Satan] in his jahl [ignorance], and thus falling into shirk [opposite of Tawhid]".^[78]

In many jurisdictions of the world, the laws and the people's general attitude hold that the sphere of public life should be secular^{[citation needed]} and that belief in and the practice of religion should remain in the sphere of private life.^{[citation needed]} One motive for adopting that stance has been to reduce the effects of conflicts between followers of different religions or between adherents of secularism and those of a religion.^{[citation needed]} In public life, that view insists that the authority of the state prevails over any religious authorities.

For some Islamic thinkers, these propositions infringe the doctrine of tawhid and are therefore anathema. If the cosmos is a unified and harmonious whole, centred on the omnipotent and omnipresent God, they hold that recognising any other authority as superior is wrong. According to one writer, "Traditionally, a Muslim is not a nationalist, or citizen of a nation-state; he has no political identity, only a religious membership in the Ummah. For a traditional Muslim, Islam is the sole and sufficient identification tag and nationalism and nation-states are obstacles".^{[79][better source needed]} Hence the idea of creating a wholly Islamic state, or a revived caliphate.

In practice, nearly all Muslims^{[citation needed]} live their daily lives under some national jurisdiction and accept at least part of the constraints this involves.^{[citation needed]}

The desire to preserve the unity and the transcendence of God has led to the prohibition of Muslims from creating representation or visual depictions of God, or of any Prophet including Muhammad. Representation in art of the human form is a disputed matter in fiqh. The key concern is that the use of statues or images may lead to idolatry. As a result, the dominant forms of expression in Islamic art became calligraphy and arabesque.^[72]

• Glossary of Islam
• Outline of Islam
• Index of Islam-related articles
• Divine simplicity
• Index finger in Islam
• Islamic view of the Trinity
• Parable of those who associate partners with God

• Modalistic Monarchianism
• Shahada
• Shirk
• Taghut
• Taqarub
• Brahmo Samaj
• Salvation
• Unitarianism