faithless

modif.

1. Not keeping faith

   disloyal, deceitful, false, traitorous, treacherous, perfidious, unreliable, untrustworthy; see also dishonest 1, 2, false 1.

2. Unbelieving

   agnostic, skeptical, dubious; see atheistic.

faithless implies failure to adhere, as to an oath or obligation a faithless wife; false, in this connection more or less synonymous with faithless, stresses failure in devotion to someone or something that has a moral claim to one's support a false friend; disloyal implies a breach of allegiance to a person, cause, institution, etc. disloyal to one's family; traitorous strictly implies the commission of treason; treacherous suggests an inclination or tendency to betray a trust his treacherous colleagues; perfidious adds to the meaning of treacherous a connotation of sordidness or depravity a perfidious informer