Here it will be well to remark, what is unanimously admitted by all theologians, even of the rigorist school, that persons who have during a considerable period of time been leading a virtuous life, and live habitually in the fear of God, whenever they are in doubt, and are not certain whether they have given consent to a grievous sin, ought to be perfectly assured that they have not lost the divine grace; for it is morally impossible that the will, confirmed in its good purposes for a considerable lapse of time, should on a sudden undergo so total a change as at once to consent to a mortal sin without clearly knowing it; the reason of it is, that mortal sin is so horrible a monster that it cannot possible enter a soul by which it has long been held in abhorrence, without her being fully aware of it.
--St. Alphonsus Liguori, The Holy Eucharist
