Lacking faith; lacking belief in something. / Not believing in God, religion, or a comparable ideology. / Unfaithful (said of people, towards their partners) / Not observant of promises or covenants.

The quality of being faithless.

plural of faithlessness

(US politics) A delegate selected to participate in an electoral college and assigned to a particular candidate who votes for a different candidate, usually to protest the already-determined outcome of the vote.

plural of faithless elector

In a faithless manner