Muhammad Mustafa [saw] is the Prophet whose
name is next to the Name of God in the Kalima.
The Prophets that came before Muhammad [saw]
brought parts of the religion of Islam,
and the religion of Islam was completed
with the Prophethood of Muhammad [saw].
Thus, whenever a non-muslim passes away,
he or she will also go thru the same procedure
in the grave as a muslim would. The heaven
is the same, the hell is the same.
The angels will say "Bismillah-ar-Rahman-ar-Raheem",
and both the non-muslim and the muslim,
whatever their native language will understand
it, and also other things. The language spoken
is the language of the soul.