: of, relating to, or characterized by faithlessness or disloyalty : treacherous
First things first—
treacherous : characterized by faithlessness or readiness to betray trust; deceptive, untrustworthy, or unreliable
I see a slight variance here. PERFIDIOUS seems to describe the effects of said behavior—how you might call someone after they have committed an act of disloyalty, or a way to describe the act itself. Treacherous has an air of intent; if one is treacherous, he is untrustworthy to the bone, born as such and bound to this definition. PERFIDIOUS strikes me as a close companion to words like despicable or hooligan. It doesn't propound the same inherent maliciousness that oozes out of a word like treacherous. Treacherous just sounds more threatening.
All that aside, I'm curious about how faithlessness translates to untrustworthiness. Beware of the non-believer. Faith has been a constant struggle for me; I grew up without religion, out from under the umbrella of an ultimate authority. I was always a skeptic, never entirely believing in anything without question, coming close only to the idea of "self"—a concept in which family and inspirational children's book suggested I should believe.
Faith, to me, is a concept that is earned, not just blindly granted. Thirty years in, I have found I have faith in a handful of people in my life. I have faith in my judgment calls. I have faith in unpredictability. I have faith in the earth's counter-clockwise rotation, and if the earth decided to be unpredictable, I might have faith in that as well. In general I have faith in more than I give myself credit for.
Regardless, I don't feel that my faith, or lack thereof, has any bearing on whether or not I'm a loyal or trustworthy person. I can be loyal to something I don't have faith in. Case in point: Whole Foods. I've worked for this company for four years. It's a decent company. They do some good things. I never bought into the idea that WF is the end-all be-all of philanthropy in the world of food service. But that doesn't mean I'm a disloyal or unreliable employee. It just means I'm realistic. In fact, I think a blindly faithful employee would be more dangerous; one who never questions will ultimately be crestfallen when he discovers the truth of imperfection. That blow might send a believer to the opposite end of the spectrum, to desperate acts of PERFIDIOUSNESS. I, on the other hand, have accepted WF's flaws and stayed, henceforth there is little room for reactionary behavior.
Maybe I am misinterpreting my faithlessness for a different kind of faith—a faith in the natural fallibility of everything. This seems to me a more solid ground to stand on, or perhaps, less of a distance to fall.