showing or constituting penance; "penitential tears"; "wrote a penitential letter apologizing for her hasty words"

A penitential is a book or set of church rules concerning the Christian sacrament of penance, a "new manner of reconciliation with God"Rouche 1987 p. 528. that was first developed by Celtic monks in Ireland in the sixth century AD. When priests heard confessions, a practice originally begun by monks to use amongst themselves as a religious cleansing ritual, they compiled unofficial handbooks that dealt with the most confessed sins and a wrote down set penances for those sins.