nevernik

1. (of religion) unbeliever; infidel
   Za njih je svako ko ne ide u crkvu nevernik. — To them, anyone who doesn't go to church is an unbeliever.
2. humor. doubter; doubting Thomas
   E, nevernici, sad vidite da sam bio u pravu! — Well, you doubters, now you see I was right!
3. (of a partner) unfaithful partner; cheater
   Saznala je da joj je muž nevernik i tražila razvod. — She found out her husband was a cheater and asked for a divorce.

The feminine form is nevernica.