A better word for muslim would be “infidel” in sense 1 (but also 2a and 3) below, although muslims use “infidel” for those not believing in their particular branch of islam. Although both pagan (sense 2) and infidel (sense 2b) fit me, “irreligious” would be a better word. But I prefer atheist, as it conveys the meaning of non-belief better, imho.
Definition of pagan
(Entry 1 of 2)
1 : heathen sense 1 especially : a follower of a polytheistic religion (as in ancient Rome)
2 : one who has little or no religion and who delights in sensual pleasures and material goods : an irreligious or hedonistic person
Contrast with infidel:
Definition of infidel
(Entry 1 of 2)
1 : one who is not a Christian or who opposes Christianity
2a : an unbeliever with respect to a particular religion
b : one who acknowledges no religious belief
3 : a disbeliever in something specified or understood