Tawhid, which is amongst the most prominent principles of all Abrahamic faiths, means to believe there is only one god, and is the opposite of polytheism (shirk). Tawhid also includes denying any parallel, resembling being or any sort of imaginative or physical composition for God.
Many arguments have been presented to prove God’s oneness, such as: the arguments of wahdah, design, oneness of prophets, denial of parallel, God’s needlessness, God’s non-composition and transcendence. However, what can help us the most in this regard is to know that God is absolute existence and absolute existence, due to its nature, has no limits or confining boundaries. If there is a parallel or partner for this absolute existence, each will have an independent existence, meaning that each one would lack the existence of the other, and this clearly contradicts the fact that they are absolute and unlimited. Therefore, it is logically impossible to have more than one God.
“Tawhid” is the most prominent principle of Islam and other Abrahamic faiths. Tawhid literally means to unify and, when used as a theological term, means to believe in one god. The opposite of tawhid is shirk, which is referred to by the Quran as the “great oppression” or the “unforgivable sin”.
Tawhid has different levels and degrees, namely: Dhati (in essence), Sifati (in attributes), Afaali (in actions), Ebadi (in being worshipped), Malikiyyah (in ownership), Haakimiyyah (oneness in ruling) and oneness in legislation. Below we will only explain tawhid dhati and the arguments that prove it.
Dhati Tawhid (Oneness in Essence):
This level of tawhid is divided into two parts: Tawhid Ahadi and Tawhid Wahedi. The first means that there is only one being whose existence is necessary and that it has no partner or parallel. The second means to deny any form of intellectual, actual or imaginative composition for God and to believe in his non-composition, meaning that God is not a wajib al-wujud (a being whose existence is necessary) that simply does not have a parallel and is the only instance of wajib al-wujud to be found, but rather a wajib al wujud that cannot have a parallel, as it is impossible to picture a partner for him. In philosophy, this oneness is referred to as al-Wahdah al-Haqqah al-Haqiqiyyah (true and real oneness) as opposed to oneness in number, gender, etc.
Arguments in Favor of Tawhid Dhati:
Many arguments can be presented to prove Tawhid Dhati, some of which the Quran makes reference to. The following is an explanation of some of these arguments:
1- The argument of one power controlling and managing the world: If there were two powers in the world, there would be evident chaos and disorder in the world. Therefore, the fact there is order in this world shows that there is only one god. Even though this argument solely proves the oneness of a manager for this world, but it can also prove the oneness of God.
2- The argument of the unified message of prophets: If there was more than one god, the other gods would also send prophets to guide people, but such a thing has never taken place. In other words, the fact that there are no prophets sent by the other gods shows that they (other gods) do not exist. 
3- The argument of denying all parallels and partners: God’s essence is unlimited and transcendental and possesses all excellencies. There is no place that is empty of Him, no time that He is not present in and no perfection that He does not possess. He has no limits as the Quran says: “هُوَ اللَّهُ فِی السَّماواتِ وَ فِی الْأَرْضِ” “فَأَیْنَما تُوَلُّوا فَثَمَّ وَجْهُ اللَّهِ” He is everywhere and is never confined to limits, including time and place. Knowing that wajib al-wujud is absolute existence and has no limits, He cannot have any parallels or partners that both share something with Him in their essence and also have something that sets them apart from Him, for His absoluteness in existence will not leave any room for another god.
4- The argument of God’s needlessness: It is proven that God has no needs. Denying need from one, is another way of denying all composition from him, because every comprised being is in need of its parts to exist. Therefore, by proving that God has no needs, we can prove his oneness in the sense that He has no composition and is not comprised of different parts.
5- The argument of God’s non-composition and being unlimited: As we explained above God has no parts and no needs. He is absolute existence and, due to his absoluteness, nothing limits His existence. If there was another wajib, each one would possess different existences and lack the existence of the other. Therefore, their existence would be limited and both gods would be comprised of existence and non existence which is the worst of compositions. As a result, no mumkin al-wujud (contingent being) can be parallel to God because of the mumkin’s limited existence and because God’s unlimited existence does not leave any room for any parallel.
 Luqman:13 “وَ إِذْ قالَ لُقْمانُ لاِبْنِهِ وَ هُوَ یَعِظُهُ یا بُنَیَّ لا تُشْرِکْ بِاللَّهِ إِنَّ الشِّرْکَ لَظُلْمٌ عَظیمٌ”
 Nisa’:48 “إِنَّ اللَّهَ لا یَغْفِرُ أَنْ یُشْرَکَ به وَ یَغْفِرُ ما دُونَ ذلِکَ لِمَنْ یَشاءُ”
 See: Anbiya:22; Mu’minun:91.
 See: Ahqaf:4; Anbiya:25.
 Nahjul-Balaghah, letter 31.
 See: Makarim Shirazi, Nasir, Aqideye Yek Mosalman, Hadaf Press, first edition, pg. 38.
 See: Makarmi Shirazi, Nasir, Khoda ra Chegune Beshnasim, Mohammadi Press, 1343, pg. 42.
 Makarim Shirazi, Nasir, Aqideye Yek Mosalman, pg. 43.
 See: Surah Tawhid; Shura:11.
 There are verses in the Quran that mention God’s needlessness, such as: “وَاللَّهُ هُوَ الْغَنِیُّ الْحَمیدُ” Fater, 15
 Jawadi Amoli, Abdullah, Sharhe Hekmate Mota’aliyeh (Asfare Arba’ah), vol. 6, section 1, pp. 433-434, al-Zahra Press.
 For further information, see index: Assuming a mumkin al-wujud next to a wajib al-wujud, Question 80 (website: 1862) and index: The endlessness of God’s existence, Question 1944 (website: 1944).