“….we arrive at a theological concept of mortal sin and of its essential difference from venial sin. A person commits a mortal sin when he abandons the divine order so as to give up God as his last end and withdraws from subjection to Him. He commits a venial sin when he does not carefully pursue the line of conduct leading to his goal, but at the same time does not abandon the goal itself and consequently his union with God. Venial sin is a violation of the order leading to the end, but not of the end itself. Therefore charity, as love of the end, and with it grace, is not extinguished by venial sin. Venial sin defiles grace in the way that gold is exteriorly tarnished without any interior decomposition. That is why charity and grace are not substantially diminished by venial sin. Only mortal sin can assail their substance, and when it does so, it not merely weakens but destroys.”
-Matthias Scheeben in Nature and Grace.