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.
