faithless

adjective6

1. •
   Lacking faith; lacking belief in something.

   • "You're so faithless about pills," he said, "that I'm not going to give you any." — 1970
2. •
   Unfaithful (said of people, towards their partners)

   • Menelaus, who fought to recover his faithless wife, has clearly rooted himself in Sparta for the remainder of his life […] — 1996
3. •
   Not observant of promises or covenants.
4. •
   Not true to allegiance, duty, or vows
5. •
   Not believing in God, religion, or a comparable ideology.
6. •
   Serving to disappoint or deceive