From Longman Dictionary of Contemporary Englishunbelieverun‧be‧liev‧er /ˌʌnbəˈliːvə $ -ər/ noun [countable] RRsomeone who does not believe in God or a particular religion
Examples from the Corpus
unbeliever• Among all the saints sat an unbeliever.• She could make even the unbelievers know that she had drunk from the Spiritual fountains of water. 