Origin 610 to 632 CE
Tawhid asserts that God is one, without partner, without division and without likeness. It is the first clause of the confession of faith and the point on which Islam most explicitly distinguishes itself from Christianity, whose trinitarian doctrine the Quran addresses and rejects. Its negative counterpart is shirk, associating anything with God, which is named as the one sin not forgiven if a person dies in it.
The doctrine sounds simple and generated centuries of difficulty, because the Quran also speaks of God as hearing, seeing, knowing, and as seated on a throne. If those attributes are real and eternal, do they compromise the unity? If they are not real, what does the text mean?
The Mu'tazila answered that the attributes are not distinct from the essence, and drew the further conclusion that the Quran is created rather than eternal, since an eternal uncreated speech alongside God looked to them like a second eternal thing. Under caliphal patronage this position was enforced, and scholars who refused it were interrogated and imprisoned; Ahmad ibn Hanbal's refusal under that pressure is one of the formative episodes in Sunni memory. The policy was abandoned and the opposite view prevailed: the attributes are real, the Quran is uncreated, and the manner is not to be interrogated. The Ash'ari school's formula for the anthropomorphic language, affirm it without asking how, became the mainstream Sunni settlement, with the Maturidi school close alongside.
A later scheme divides tawhid into three: God's uniqueness as lord and creator, his uniqueness as the sole object of worship, and the uniqueness of his names and attributes. This tripartite division is strongly associated with Ibn Taymiyya and with the Salafi tradition that draws on him, and it does real polemical work, since it allows the charge that people who acknowledge God as creator may still fail in the worship category. It is widely taught but it is not a universal or uncontested Islamic framework, and describing it as the Islamic doctrine of tawhid without attribution takes one school's formulation for the whole.