" . . . religious faith is necessarily subjective, being incommunicable by any kind of proof or evidence."
The same could be said for matters of conscience in general."

Not at all. As far as memory serves, contemporary moral philosophers will usually use non-religious reasoning in their arguments.

"Conscience" IMO is pretty much founded on a biological mammalian instinct to care for and support the group (including 'self sacrifice'), the finer details of which tend to be something we learn in order to integrate successfully with our society, however it happens to have developed.