Divine Unity refers to the belief that Allāh is One. He is not composed of parts and attributes, because any composed being is in need of parts and the one who composes it. Also, it is impossible for the being that is in need to give existence to itself, let alone give existence to one other than itself. Divine Unity is also the belief that Allāh has no partners in Divinity or in His attributes.1 1 There are stages of Divine Unity. We will briefly mention them: Unity of Essence: Every being is composed except the Holy God. Indeed, He is the One for Whom Oneness is a part of His Essence. Everything other than Him can be divided either in existence, like the body to matter and shape; or in imagination, like the time into moments; or in the mind, like the human being into humanity and existence. The division of every existing being has a limited end. However, Allāh is unlimited: Say: He Allāh is One. Shurayḥ ibn Hānī reports on the authority of his father saying: On the Day of the Battle of Jamal, a Bedouin stood before the Commander of the Faithful (A), and asked, “O Commander of the Faithful! Do you say that Allāh is One?” He says: the people attacked him saying that “O Bedouin! Do you not see the state of mind of the Commander of the Faithful?” So the Commander of the Faithful (A) said: Leave him alone. Verily, what this Bedouin seeks to know is what we are seeking to establish by fighting these people. He (the Commander of the Faithful) then said: O Bedouin! The statement that ‘Allāh is One’ is of four types: Two types cannot be applied to Allāh, the Mighty and High; while the two other types can be applied to Him. As for the two types of statements that cannot be applied to Him, the first is the claim that Allāh is One in a numerical sense. Such a statement cannot be applied to Allāh, because as the One, He has no second, and He is not subject to mathematical calculation. Do you not see that he who claims that Allāh is the third of the three has disbelieved (in Him)? The second statement which cannot be applied to Allāh is the claim that He is One with humankind. Since He derives the species from the genus, He cannot be described through anthropomorphism [tashbīh]. Our Lord is Higher and more Exalted than this. As for the two types of statements that can be applied to Allāh, the first would be to say that He is the One who resembles none. This is a proper description of our Lord. The second statement which can apply to Allāh is the statement that He, the Mighty and High, is Alone, in the sense that He is Indivisible in Existence [wujūd], Intellect [ᶜaql], and Thought [wahm]. That is another dignified description of our Lord, the Mighty and High. (Shaykh Ṣadūq, The Book of Divine Unity: pp 83, ch. 3, ḥadīth 3). Unity of Essence and Attributes: The meaning of this is that His Attributes of Essence, like Ever-Living, Omniscience and Omnipotence, are a part of His Essence. Otherwise, separation of essence and attributes would necessitate composition and division. The composed of components needs parts, and also needs one who composes it. If the attributes were in addition to the essence, then it would necessitate that the essence is vacant from the attributes of completion. From another angle, if the attributes were in addition to the essence, it would also necessitate that the essence would be vacant from the attributes of completion and the attributes will be joined to possible being. In fact, this would necessitate that His essence also be possible being, because the one vacant of attributes of completion and carrying possibility, requires self-sufficiency in its essence. The Commander of the Faithful (A) said: The best of service is to acknowledge the existence of Allāh. The foundation of acknowledgement of Allāh is to profess His Unity. Allāh’s Unity is established through the denial of descriptions, since all intellects bear witness that descriptions are created, and whatever is created requires a Creator who is neither identifier or identified. Every identifier and identified is connected, and connection is evidence of occurrence. (Shaykh Ṣadūq, Divine Unity: pp 34, ch. 2, ḥadīth 2). Unity of Divinity And your God is One God! There is no god but He; He is the Beneficent, the Merciful. (Qur’ān: 2:163). Unity of Lordship Say: What! Shall I seek a Lord other than Allāh? And He is the Lord of all things. (Qur’ān: 6:164). O my two mates of the prison! Are sundry lords better or Allāh the One, the Supreme? (Qur’ān: 12:39). Unity of Creating Say: Allāh is the Creator of all things, and He is the One, the Supreme. (Qur’ān: 13:16) And those whom they call on besides Allāh have not created anything while they are themselves created. (Qur’ān: 16:20). Unity of Service Say: Do you serve besides Allāh that which does not control for you any harm, or any profit? (Qur’ān: 5:76). Unity of Command and Order Surely His is the creation and the command; blessed is Allāh, the Lord of the worlds. (Qur’ān: 7:54). Judgment is only Allāh's. (Qur’ān: 12:40). Unity of Fear But do not fear them, and fear Me if you are believers. (Qur’ān: 3:175). Therefore fear not the people and fear Me. (Qur’ān: 5:44). Unity of Sovereignty And say: (All) praise is due to Allāh, Who has not taken a son and Who has not a partner in the kingdom. (Qur’ān: 17:111). Unity of Benefit and Harm Say: I do not control any benefit or harm for my own soul except as Allāh please. (Qur’ān: 7:188).

We will now explain some proofs of Divine Unity:
First Proof:
If there were numerous gods then for each one of them to be a god, it would require them to share Divinity. Each one of them requires distinction from the other to confirm duality. Thus, each one would be composed from what they both share and in what they both differ. Second Proof: Existence of more than one god, without distinction, is impossible. Distinction is a cause of absence of what the other god may have of perfection. The one lacking perfection is in need. Therefore, the chain of need must end at the Self-Sufficient in essence from all dimensions. Otherwise, it would necessitate for all possible beings to be nonexistent, because the one that does not possess existence, cannot give it. Say: Then who can control anything for you from Allāh if He intends to do you harm or if He intends to do you good. (Qur’ān: 48:11). Unity of Sustenance Say: Who gives you the sustenance from the heavens and the earth? Say: Allāh. (Qur’ān: 34:24). Or who is it that will give you sustenance if He should withhold His sustenance? (Qur’ān: 67:21). Unity of Reliance And rely on Allāh; and Allāh is sufficient for a Protector. (Qur’ān: 33:3). Allāh, there is no god but He; and upon Allāh, then, let the believers rely. (Qur’ān: 64:13). Unity of Intention of Deeds And noone has with him any boon for which he should be rewarded, except the seeking of the pleasure of his Lord, the Most High. (Qur’ān: 92:19-20). Unity of Honour This stage is for the ones who witness the destruction of the universe and its perishing. They fully comprehend: Everything is perishable but He, (Qur’ān: 28:88) everyone on it must pass away, and there will endure forever the person of your Lord, the Lord of Glory and Honour. (Qur’ān: 55:26-27). The Divine Unity of Honour in nature manifests into the Divine Unity of Honour in choice through their striving for Allāh. Then it would be confirmed what has been entrusted in the souls from the knowledge and the free-will without their free-will. Hence they reach what the Exalted has said: And the faces shall be humbled before the Living, the SelfSubsistent. (Qur’ān: 20:111), up to the Word of the Exalted: Surely I have turned myself, being upright, wholly to Him Who originated the heavens and the earth. (Qur’ān: 6: 79).

Third Proof:
Indeed, Allāh, the Exalted, is present without limitation. As ᶜAlī (A) says: Limitation and end do not apply to Him,1 because every limited being is composed of essence and the limitation of that essence. The limitation of essence would mean absence of perfection, which is in addition to that essence. Thus, this composition is the worst of its types, as composition can be between two existing things or between one existence and one nonexistence. The supposed composition would be between an existence and a nonexistence. However, all types of compositions are impossible for Allāh, the Exalted. The existence which has no limitation must be One for which a second cannot be conceived, because conceiving a second to Him would necessitate limiting Him. A limitation is composed of existence and nonexistence. Every composed is in need of what it is composed of. Therefore, association in Divinity ends in making the god in need of nonexistence. The Glorified Allāh is Unique in essence and attributes. Hence, He has no second to Him, neither confirmed nor conceived. Fourth Proof: Indeed, the unity in the system of all parts of the universe, and in the universe as a whole, proves the Oneness of the Creator. A close examination of the system and composition of every part of the categories of the universe, and their connection with each other, reveals that both parts of the universe and the universe as a whole are created by One, Omniscient, Omnipotent and All-Wise Creator. Indeed, the formation of the parts of a tree, the parts of the body of an animal and its strength, their connection with each other, their connection with the Earth and the sun, the connection of the solar system with other solar systems and the galaxy, in one sentence is: Verily, from the composition of an atom and what goes on around it to the composition of the sun and the galaxies, reveal that the Creator of all is One: And He it is Who is God in the heavens and God in the earth; and He is the Wise, the Knowing.2 O men! Serve your Lord Who created you and those before you so that you may guard (against evil). Who made the earth a resting place for you and the heaven a canopy and (Who) sends down rain from the cloud then brings forth with it subsistence for you of the fruits; therefore do not set up rivals to Allāh while you know.3

Reference:
1 Nahj al-Balāghah: sermon no. 186.
2 Qur’ān: 43: 84.
3 Ibid: 2: 21-22.


Fifth Proof:
Imām al-Ṣādiq (A) was asked, “Why is it not permissible that there be more than one creator of the universe?” Thus, he (A) replied: …Furthermore, if you claim that there are two gods, there must be a division between them both, which has always existed, and that division would have to be a third god. However, if you claim that there are three gods, then there must be two divisions between them, leaving us with five gods, and so on ad aeternum.1 Sixth Proof: The Commander of the Faithful (A) said to his son al-Hasan (A): Beware, O son that if your Lord had an associate then his messengers would have certainly come to you and you would have seen signs of his sovereignty and dominion. You would have learnt his actions and attributes. However, He is One God like He has described Himself.2 The outcome of the faith in Oneness of Allāh, the Exalted, is His Unity of Service, because other than Him do not deserve service and cannot attain it. As everything and everyone other than Him is His servant: There is noone in the heavens and the earth but will come to the Beneficent Allāh as a servant.3 Furthermore, service to other than Allāh, the Exalted, is humiliation from the humiliated and begging from the needy. In fact, it is humiliation even for humility and is begging from the need itself. O men! You are they who stand in need of Allāh, and Allāh is He Who is the Self-Sufficient, the Praised One.4 Verily, faith in His Oneness and that every being is from Him, due to Him and will return to Him, is summarised in three sentences: There is no god but Allāh; There is no power and no strength except Allāh; And to Allāh all things return.5 Fortunate is he who remembers these pure words constantly, and sleeps and wakes with them, and lives and dies upon them. Thus, he will succeed in reality: Surely we belong to Allāh and to Him we shall surely return.6 The Effect of Divine Unity on Man and Society Amongst the effects that occur when mankind reaches the reality of Divine Unity, is that the thinking and intent of an individual and society must....

Reference
1 Shaykh Ṣadūq, Divine Unity: pp 243, ch. 36, ḥadīth 1.
2 Nahj al-Balāghah: letter no. 31.
3 Qur’ān: 19: 93.
4 Ibid: 3: 15.
5 Ibid: 3: 109.
6 Ibid: 2: 156.

....focus on one goal. There is no goal higher than that. In fact, there is no other goal but: Say: I exhort you only to one thing, that rise up for Allāh's sake in twos and singly.1 Even when the focus of the rays of the human soul is on nonessential points, it can do amazing things. However, if the rays of the human soul in thinking and intent focused on one reality, which is the beginning and the end of existence, and the Light of the heavens and the earth,2 then to what stage will this man achieve? If the individual and the society reach the status, concerning which the Glorified and Exalted says: Surely I have turned myself, being upright, wholly to Him Who originated the heavens and the earth, and I am not of the polytheists,3 then through their connection with Allāh and moulding with Allāh’s ethics, they will become such a source of goodness, privilege and perfection that cannot be described. To end this discussion, we will mention the word which is a fort of Allāh for protection from the wrath of this world and the hereafter. Abū Ḥamzah reported on the authority Abū Jaᶜfar al-Bāqir (A) saying: There is nothing greater in reward than testifying that: There is no god but Allāh [Lā ilāha illā Allāh], because nothing equals Allāh, the Mighty and High, and nobody shares His Power.4 From this tradition, we conclude that just as there is no equal or associate with Allāh, the Exalted, in His Command; similarly, there is no deed equal to the testimony: there is no god but Allāh [Lā ilāha illā Allāh]. Likewise, there is nothing equal in reward for this deed. Indeed, the verbal testimony there is no god but Allāh [Lā ilāha illā Allāh], protects the soul, wealth and honour in this world. Moreover, testimony with the heart causes salvation from the wrath of Hell in the hereafter, and success to the highest level of Paradise. Thus, this sacred word is the manifestation of mercy of both the beneficence [al-ramāniyyah] and the mercifulness [alraīmiyyah]. It is reported from al-Ṣādiq (A): Verily, Allāh, Blessed and Exalted be He, has taken an oath by His Dignity and Glory that He will never punish the people who believe in His Unity with Hell-fire.5 It is reported from the Prophet (S): The one whom Allāh has blessed with belief in Divine Unity shall have Heaven as his reward.1....

Reference:
1 Qur’ān: 34: 46.
2 Ibid: 24: 35.
3 Ibid: 6: 79.
4 Shaykh Ṣadūq, Divine Unity: pp 19, ch. 1, ḥadīth 3.
5 Ibid: pp 20, ch. 1, ḥadīth 6.

...Surely whoever remembers these words constantly, the vessel of his heart will be secured with the anchor of there is no god but Allāh [Lā ilāha illā Allāh] from the abyss of destruction and the waves of fear of calamities, whispers and evils: Those who believe and whose hearts are set at rest by the remembrance of Allāh; now surely by Allāh's remembrance are the hearts set at rest.2 The word: There is no god but Allāh [Lā ilāha illā Allāh], is a remembrance that can be said loudly or quietly. Thus, it is the combination of hidden and apparent remembrance. It contains the sacred word “Allāh.” It is reported from the Commander of the Faithful (A) that it is the greatest name of Allāh, the Exalted. Say: Tell me if the chastisement of Allāh should overtake you or the hour should come upon you, will you call (on others) besides Allāh, if you are truthful? Nay, Him you call upon, so He clears away that for which you pray if He pleases and you forget what you set up (with Him).3 Abū Sa‘īd al-Khudrī reported from the Prophet (S) that he said: Allāh, Mighty be His Glory, said to Mūsā (A), “O Mūsā! If everything that exists in all the heavens, their inhabitants, and the seven earths, were placed in one side of the scale, and ‘there is no god but Allāh’ was placed in the other side, then ‘there is no god but Allāh’ would tip the scale.”4 Thus, nothing from the earth, the heavens, the material and the abstract things, can equal ‘there is no god but Allāh.’ None perceive its reverence but the ones who have reached the reality of negation and proof, as this testimony relates.

References:
1 Shaykh Ṣadūq, Divine Unity: pp 22, ch. 1, ḥadīth 17.
2 Qur’ān: 13: 28.
3 Ibid: 6: 40-41.
4 Shaykh Ṣadūq, Divine Unity: pp 19, ch. 1, ḥadīth 34.