faithless

adj

1. Lacking faith; lacking belief in something.

   “"You're so faithless about pills," he said, "that I'm not going to give you any."”

2. Not believing in God, religion, or a comparable ideology.
3. 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 […]”

4. Not observant of promises or covenants.
5. Not true to allegiance, duty, or vows
6. Serving to disappoint or deceive