St. Augustine and St. Thomas define mortal sin to be a turning away from God: that is, the turning of one's back upon God, leaving the Creator for the sake of the creature. What punishment would that subject deserve who, while his king was giving him a command, contemptuously turned his back upon him to go and transgress his orders? This is what the sinner does; and this is punished in hell with the pain of loss, that is, the loss of God, a punishment richly deserved by him who in this life turns his back upon his sovereign good.
Alphonsus LiguoriHow, then, shall God give himself entirely to that one who, besides his God, loves creatures still?
Alphonsus LiguoriCertainly amongst all devotions, after that of receiving the sacraments, that of adoring Jesus in the Blessed Sacrament holds first place, is most pleasing to God, and most useful to ourselves. Do not then, O devout soul, refuse to begin this devotion; and forsaking the conversation of men, dwell each day, from this time forward, for at least half or quarter of an hour, in some church, in the presence of Jesus Christ under the sacramental species. Taste and see how sweet is the Lord.
Alphonsus LiguoriSaint Teresa, as the Roman Rota attests, never fell into any mortal sin; but still Our Lord showed her the place prepared for her in Hell; not because she deserved Hell, but because, had she not risen from the state of lukewarmness in which she lived, she would in the end have lost the grace of God and been damned.
Alphonsus Liguori