Belief that Muhammad (SAW) is the Seal of the Prophets, that there is no Prophet after him
He was sent to the entirety of the two races - the Jinn and mankind.
Anyone who does not believe in what he came with is not a believer, let alone being one of the pious, God-fearing Awliya of Allah!
Whosoever believes in a part of what he came with and disbelieves in the remainder is also a disbeliever and in no way a believer