And so, Dearly Beloved, as it was commanded in the Law to offer tithes of all things, so let you offer Him a tithe of your days. Let each one mortify his own body, as far as his strength allows, and let him weaken its desires, and lower the pride of its evil lusts, so that he may become, in the words of Paul, a living sacrifice (Rom. xii. 1). A man is both living and a sacrifice when he has died to the desires of his body, though he has not departed from this life. It is the pleasure-loving body that leads us into sin; mortification leads us back to forgiveness.
St. Gregory the Great
(The Sunday Sermons of the Great Fathers)