Tawḥīd (Arabic: التَوحيد), or monotheism is the most fundamental doctrine in Islam, which means believing in the oneness and uniqueness of God and not ascribing any partners to Him. The message of the Prophet Muhammad (s) since the beginning of his mission was based on the call for tawhid and the rejection of polytheism. Tawhid is a major theme in the Qur'an and hadiths. Al-Tawhid is also one of the names of sura 112 of the Qur'an, which presents a brief but deep expression of the doctrine.
In Islam, tawhid stands in contrast to shirk (ascribing partners to God). Muslim theologians have counted a number of levels for tawhid: al-tawhid al-dhati (unity of the divine essence), which refers to the belief in God as the only God and negating any partners for Him; al-tawhid al-sifati (unity of the divine attributes), which refers to the identity of divine essence with divine attributes; al-tawhid al-af'ali (unity of actions), which refers to the fact that God is the only independent cause and that He needs no assistance for His actions; al-tawhid al-'ibadi (unity of worship), which refers to the fact that no one other than God deserves to be worshiped.
There are various arguments for tawhid in the Qur'an, hadiths, and the works of Muslim philosophers and theologians. The argument of tamanu' (Argument of Antagonism), the argument from appointing prophets, and the Argument of the Impossibility of Multiplicity are some of the arguments presented by Muslim theologians and philosophers for tawhid.
Some Sunnis, including Ibn Taymiyya, Muhammad b. 'Abd al-Wahhab, and 'Abd al-'Aziz Bin Baz, have regarded asking for shafa'a and tawassul as instances of shirk. Based on the teachings of the Qur'an, other Muslims reject this idea and maintain that there is no problem in asking for intercession when the intercessor is not considered divine and asking for intercession is done just to honor the intercessor and get closer to God through them.
Shiite scholars have written extensively on the topic of tawhid. Some of their works are devoted exclusively to tawhid, and others are parts of works on theology. Some of these books are al-Tawhid by al-Shaykh al-Saduq, Gowhar-i murad by 'Abd al-Razzaq Lahiji, al-Rasa'il al-tawhidiyya by Allama Tabatabai, and Tawhid by Morteza Motahhari.
Tawhid, in the sense of believing in the oneness of God, is the most fundamental Islamic doctrine, according to which the One God is the sole creator of the world and does not have any partners. Tawhid is also used, as seen in some hadiths, to mean confessing this doctrine. Sometimes the term tawhid refers to the theological discussions related to God, His attributes, and actions. According to hadiths, the Prophet (s) used the word to mean the affirmation of the proposition "lā ilāh illā Allāh, waḥdah lā sharīk lah" (لا إله إلا ألله، وحده لاشريک له, there is no god except Allah, he is one and has no partner). In response to questions about tawhid, Imam al-Sadiq (a) and Imam al-Rida (a) have brought up some theological discussions such as negating human attributes from God.
There are three different types of tawhid,
Tawhid is regarded as the most important characteristic teaching of Islam. As the Qur'an stipulates, all the prophets (a) called people to tawhid. Although the term "tawhid" is not mentioned in the Qur'an, there are many Qur'anic verses that are related to the affirmation of tawhid and negation of polytheism, such that the main purpose of the Qur'an is considered by Mulla Sadra in his exegesis to be the affirmation of tawhid.
Confessing tawhid and rejecting polytheism are among the first teachings of the Prophet (s), which he proclaimed at the beginning of his mission in Mecca. The representatives that the Prophet (s) sent to various places, such as Mu'adh b. Jabal, would first and foremost call people to tawhid. Imam Ali (a) states that believing in the oneness of God is the foundation of knowing God.
Tawhid is emphasized in the Qur'an in various words and expressions. Negating other gods, emphasizing the oneness of God for all, condemning polytheists, rejecting the Trinity, and negating anything that is similar to God are among the themes related to tawhid in the Qur'an. Some of the Qur'anic verses about tawhid are,
In Islam, tawhid is in contrast with shirk (ascribing partners to God), and thus eliminating shirk is one of the main themes and aims of the Qur'an. Just as different levels have been mentioned for tawhid, shirk also is considered to have different levels: al-shirk al-dhati is the belief in more than one God; al-shirk al-af'ali is the belief in more than one independent cause in the world; al-shirk al-sifati is the belief in the ontological multiplicity of God's attributes and their distinction from His essence, and al-shirk al-'ibadi is worshipping something other than the one God.
Unity of the essence is the first stage of monotheism, consisting in the belief in the oneness of divine essence. That is, the divine essence is not subject to plurality and has no peer.
Qur'an 112 (Sura al-Tawhid), refers to both kinds of Unity of essense: the word 'Aḥad' at the beginning of this sura refers to the simplicity of divine essence, and the last verse ('there is none like unto Him') refers to the rejection of any peers and partners about God.
Unity of the attributes is the view that the divine essence is identical with divine attributes. Thus, the divine knowledge is not distinct from divine power; all His being is both knowledge, power, and other attributes through and through, and all His attributes are identical to one another and to his essence. Contrary to other beings, in which attributes are distinct from their essence, and they gain the attributes. 
Qur'an has cleared God from the attributes. In a speech narrated by Abu Basir, Imam al-Sadiq (a) attributes divine knowledge, hearing, sight and power to divine essence and has stipulated that God has been able to hear and see before there had been anything to be heard or seen.
Unity of actions (al-tawhid al-af'ali) is the view that in order to do His acts, God does not need anything or anyone other than His own essence—He is independent in all His acts. According to the unity of actions, nothing in the world can have any effects or do anything except in virtue of the power it is given by God; all actions, movements, and effects are originated in His essence. He has no partners in His essence, He has no partners in His agency—including creation, monitoring, owning, and governing the world.
Qur'an introduces God as the creator of everything and the one, all paramount.
Imam al-Sadiq (a) has considered God as the only one who can create something from nothing.
Unity of worship is a sort of theoretical monotheism, which amounts to the thought that nothing except Allah is worthy of worshiping—that is, there is no god except Allah. Therefore, every person has to worship God and on the basis of His commands. According to Qur'an inviting to worship the One God is the primary mission of all of the prophets. Monotheism in worship has been referred to by some Qur'anic verses, such as Qur'an 6:102, Qur'an 1:2-5, Qur'an 21:25.
This is taken to be implied by other types of monotheism: if our existence is from Allah, our survival depends on Him, the only truly independent cause in the world is Allah, and it is only Allah who has the right to command and legislate, then no one else is worthy of worship. Any non-divine motivations involving in worships reduces their value and it sometimes leads to their religious invalidity.
In the Qur'an and hadiths and in the works of Muslim philosophers and theologians, a number of arguments have been mentioned for tawhid. Some of these arguments are as follows:
If there were two gods and one of them willed to do something and the other willed to do its opposite, one of the following would happen:
Other arguments have also been presented by Muslim theologians and philosophers for the oneness of God, such as the Argument of Specificity (ta'ayyun), the Argument of the Impossibility of Multiplicity, the Argument of the Multiplicity of the Objects of Power, and the Argument from Appointing the Prophets. The latter is taken from Imam Ali's (a) letter to his son Imam al-Hasan (a), in which it is stated that if God had any partners, they would send their prophets to people.
Wahhabis regard the Shiite practice of asking the Prophet (s) and Imams (a) for intercession and seeking the blessings of their graves and relics as shirk. However, the Shi'a reject this accusation and believe that those who do so never intend to worship these friends of God and do not believe in their divinity. Their intention is only to honor those righteous servants of God and to get closer to God by honoring them.
According to Ibn Taymiyya, whoever asks Imam Ali (a) for intercession is an infidel and whoever doubts the infidelity of such a person is also an infidel. Moreover, he maintained that whoever visits the grave of the Propeht (s) or anyone else and asks them for something is a polytheist, who must be required to repent or be killed if he refuses to repent. Abd al-Aziz Bin Baz, the contemporary Wahhabi mufti, also considered asking the dead for healing or victory over the enemies as instances of the greater shirk.
Based on the Qur'an, the Shia reject the act of asking for intercession only if the one who is asked for intercession is considered as acting independently of God and His permission. It is this case that is an instance of the greater shirk in the lordship of God. In response to Muhammad b. 'Abd al-Wahhab and his followers who adduce the Qur'anic verses that criticize polytheists for seeking the intercession of their gods, Shiite scholars point to the fundamental difference between their action and the Shiite practice of asking the Prophet (s) and the Imams (a) for intercession, which is the fact that, unlike the polytheists, Shiites do not believe in the divinity of the Prophet (s) or the Imams (a) and do not regard them as independent agents of intercession.
Shiite scholars have authored many works related to the topic of tawhid. These works include the ones that are exclusively on the topic of tawhid and the ones that discuss this topic along other beliefs and doctrines. Some of these works are as follows: