An Unbeliever

is,

A person who has
not received Jesus Christ
as his or her Lord and Savior;
a person without
Christian faith.
(see also:'Believer')

Be ye not unequally yoked together
with unbelievers:
for what fellowship hath
righteousness with unrighteousness?
and what communion
hath light with darkness?

2 Corinthians 6:14