Origin 1469 to 1604
Ik Onkar stands at the head of the Guru Granth Sahib, and every other Sikh claim is measured against it. What follows in the Mul Mantar is largely a series of refusals: nirbhau and nirvair, without fear and without enmity, which dismantles the whole apparatus of propitiating a deity who might be angry; akal murat, timeless in form; ajuni, unborn; saibhan, self-existent.
Ajuni carries the weight. A being that is never born cannot descend into a body, and Sikh teaching therefore rejects avatarvad, the doctrine of divine incarnation, without leaving a negotiating position. Guru Arjan puts it at the limit of politeness in Bhairo: let that mouth be burnt which says that the Lord is subject to birth. Read alongside the equally flat rejection of image worship, this closes off at once the two commonest ways of relating to a deity in the surrounding religious landscape.
The oneness is not, however, the oneness of a God set over against the world. Sikh theology holds the one as both nirgun, without attributes and beyond description, and sargun, present and knowable in creation, and these are not two Gods or two stages but one reality described from two directions. That combination is what makes the entry hard to file. It is not the transcendence of Islamic tawhid, which does not let God be immanent in creation in this way; and it is not the non-dualism of Advaita, because the Sikh one is a creator who remains distinct from what is created.
What follows is visible in any gurdwara. There is no image and no shrine to a form, the focus of the room is a text, and the one is addressed by many names drawn from every tradition within reach, Ram, Hari, Allah, Khuda, Gobind, on the stated ground that the names are many and the named is one.