One who lacks belief or faith, especially in a particular religion; a nonbeliever.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unbeliever" in a sentence
"I guess the difference between a believer and an unbeliever is that an unbeliever doesn't understand how a believer thinks or how prayer or personal revelation works."
"The unbeliever is the believer's secret sharer, and vice versa."
"The same thing when done by the believer, and when done by the unbeliever, is not the same thing [Bengel]."