Tawḥīd

In plain terms

Tawḥīd is usually translated as monotheism, and that is too weak. It does not say there is one God rather than several. It says God is one in a way that admits no division, no partner, no composition and no comparison.

The consequences are strict. If God has a distinct attribute of knowledge, then God is composed of essence plus attribute, and composition is what tawḥīd rules out - which is why the question of the divine attributes occupied Islamic theology for centuries and split it.

And because nothing else is like God, ordinary language cannot be applied literally. The Qur'an describes God as hearing and seeing; taking those as human faculties compromises unity, and emptying them entirely leaves the text saying nothing.

The whole of kalām sits in that difficulty.

As a philosopher would put it

The doctrine is standardly analysed into the oneness of lordship, of divinity in worship, and of the names and attributes, with shirk - associating anything with God - as the cardinal violation.

The attribute controversy is the philosophical centre. The Muʿtazila held that affirming eternal attributes distinct from the essence introduces multiple eternals, so the attributes are identical with the essence - and drew the consequence that the Qur'an, as God's speech, is created. The Ashʿarites affirmed the attributes as real and not identical with the essence, holding them bi-lā kayf, without asking how, and the miḥna of the ninth century was fought over exactly this.

Ibn Sīnā's necessary existent is simple by argument: what is composed depends on its parts and so is not necessary in itself.

Ibn ʿArabī's waḥdat al-wujūd, the unity of existence, takes it further - there is only one existence and creatures are its determinations - and was attacked as pantheism by critics including Ibn Taymiyya.

What is actually disputed

How the divine attributes can be affirmed without compromising unity.

The Muʿtazilī answer - identifying attributes with the essence - protects unity and makes the Qur'an's descriptions difficult to take at their word, and its created-Qur'an consequence was decisively rejected.

The Ashʿarite answer - attributes real, distinct, and affirmed without asking how - keeps the text and declines to explain, which critics regard as naming the problem rather than solving it.

The philosophers' route, through Ibn Sīnā's necessary existent, secures simplicity by argument and was attacked by al-Ghazālī for making God's action necessary rather than free.

And Ibn ʿArabī's unity of existence is either the doctrine's fullest expression or its dissolution into monism, depending on who is describing it - which is a dispute internal to Islam and still live.