mortal sin

"For a single mortal sin, inasmuch as it is an offense against God, and because thereby the sinner implicitly places his chief good and end in the creature, which he loves so as to prefer it to God, and so takes away the honour of the Deity, such a sin is therefore as it were Deicide and Christicide, and so contains within it an infinity of wickedness. For it is an offense and an injury against God, who is immense and infinite. Wherefore by no punishment or penance of any creature whatsoever can just and adequate satisfaction be made to God."

"Yea, even if all men and all angels were, of their own accord, to endure all the torments of hell for eternity, they could never offer worthy penance and satisfaction to God for a single mortal sin. Christ alone can do this, inasmuch as He is the Son of God, and very God. His penance, therefore, and satisfaction, as regards to His Person, which is of infinite dignity, and are likewise of infinite value, and are equal and adequate to the infinite offense committed against an infinite God. Such is the sinfulness of sin, which if men thoroughly perceived, surely they would sin no more."

from The Great Commentary by Cornelius a Lapide