This affirmation of divine oneness is the first and most fundamental of Sikh beliefs, held by all Sikhs and inscribed at the head of their scripture. Its insistence on a formless, non-incarnate God distinguishes Sikh theology both from the image-worship and avatars found in much of the surrounding Hindu tradition and, in its own idiom and lineage of Gurus, from the different exclusivism of Islamic monotheism, positioning Sikhism as a distinct revelation rather than a synthesis or reform of either. The number one written before the word for God expresses that unity is not merely a doctrine among others but the ground of the entire faith, from which its worship, ethics, and community flow.
The line of ten human Gurus that began with Nanak closed in the early eighteenth century when Guru Gobind Singh, the tenth Guru, declared the scripture itself, the Guru Granth Sahib, to be the eternal and final Guru after him, ending the line of living human Gurus. Because the scripture opens with Ik Onkar and the Mul Mantar, that closing act bound the doctrine of divine oneness permanently to the text Sikhs revere as their living spiritual authority, so that engaging the scripture at all begins, for every reader, with this same affirmation of the one formless God.