sanctus

Sanctus, sancta, sanctum began as the perfect participle of sancio, "to ratify, make inviolable by law", so the literal force is "made binding, hedged about, not to be touched". Classical Latin uses it of temples, treaties, laws and blameless men.

In this corpus it is overwhelmingly the church's word. Sanctus Petrus is Saint Peter, and the masculine plural sancti is a noun in its own right, "the saints". The distinction worth keeping is against sacer: sacer marks a thing as made over to a god, sanctus marks it as protected and morally clean. Ecclesiastical Latin lets the two blur, classical Latin does not. Note the vocative sancte, used constantly in prayer. English took sanctity, sanctuary, sanction and sanctimonious, and the abstract noun is sanctitas.