Sikhism's foundational formula is not an invocation of a deity but a description of one, and it is the first thing in the scripture.
It runs through a sequence of terms: Ik Onkar, one being; sat nam, whose name is truth; karta purakh, the creator; nirbhau, without fear; nirvair, without enmity; akal murat, timeless in form; ajuni, unborn; saibhang, self-existent; gur prasad, known by the Guru's grace. The whole is a compressed theology, and each term is doing negative work: without fear rules out the capriciousness of a god who must be placated, without enmity rules out a god with enemies, unborn rules out incarnation.
That last one is pointed. It is a direct rejection of the doctrine of divine descent central to the Vaishnava traditions around Nanak, and it is placed among the first words of the book.
The numeral at the head is not decoration. Writing the figure one before the word makes the assertion of oneness unmistakable and unpronounceable as anything else, and it is the reason the opening is written as a distinctive symbol rather than only spoken.
The vocabulary draws on both Sanskritic and Perso-Arabic stock, which is a feature of the north Indian devotional language of the period rather than evidence of an attempt to combine two religions. The composition is Nanak's, dating within his lifetime of 1469 to 1539, and it stands at the head of the Japji Sahib, which stands at the head of the Guru Granth Sahib.