faithless

Short definition from Wiktionary

Adjective

1. 1
   Lacking faith; lacking belief in something.
2. 2
   Not believing in God, religion, or a comparable ideology.
3. 3
   Unfaithful (said of people, towards their partners)
4. 4
   Not observant of promises or covenants.
5. 5
   Not true to allegiance, duty, or vows
6. 6
   Serving to disappoint or deceive