A disbeliever, a denier: someone who rejects or disbelieves in God as per Islam (Arabic: الله Allāh) or the tenets of Islam; or more broadly any non-Muslim.
misbeliever
noun
Someone who holds a bad or wrong belief; a heretic, an unbeliever.
How often have the words kafir and misbeliever occurred in a corpus of books? (source: Google Ngram Viewer )