Tawhid is attested at the centre of Islam's own foundational text rather than at its margins. The Quran returns to the claim of God's absolute oneness across every one of its chapters in one form or another, and the shahada, the declaration that there is no god but God and Muhammad is His messenger, is both the first of Islam's five pillars and the sentence a convert recites to enter the faith; no comparably central declaration exists anywhere else in the tradition's devotional life. Islamic Creeds, W. Montgomery Watt's scholarly survey of the classical creedal literature, traces how successive generations of Muslim theologians, Ash'ari, Maturidi and others, worked out the precise philosophical shape of that oneness across centuries of disputation, arguing among themselves over questions such as whether God's attributes are identical to His essence, without ever treating the underlying oneness itself as open to revision.
The Quran's own account of pre-Islamic Arabia describes a society that worshipped multiple deities alongside a supreme god called Allah, and Islam's rise is presented, both in the Quran and in the classical Islamic historical tradition, as the restoration of Abraham's original monotheism against that later polytheism rather than as an innovation. Modern historical scholarship on pre-Islamic Arabian religion examines the same transition from outside the tradition's own theological framing, but the tradition's own self-understanding, attested continuously from the Quran itself through fourteen centuries of Islamic theology to the present day, is unambiguous: one God, without partner, without division and without equal.