St. Alphonsus Liguori - Necessity of Mortification in General (from Dignity and Duties of the Priest)

"God made man right." (Ecclesiastes 7:30) Sin came and deranged the beautiful order that God had established, and the life of man began to be a state of continual warfare. "For the flesh lusteth against the spirit: and the spirit against the flesh; for these are contrary one to another: so that you do not the things that you would." (Galatians 5:17)

For man, there are two kinds of life - the life of the angels, who seek to do the will of God, and the life of the beasts, who attend only to the indulgence of the senses.

We must always carry in our hand the mattock of mortification, to cut down the evil desires that constantly spring up and bud forth within us from the infected roots of concupiscence, otherwise the soul shall become a forest of vices.

In a word, it is necessary to cleanse the heart, if we wish for light to know God, the sovereign Good; "Blessed are the clean of heart, for they shall see God." (Matthew 5:8)

God gives the science of the saints, that is, the science of knowing and loving him, only to them that are weaned and drawn away from the breasts of the world: "But the sensual man perceiveth not these things that are of the spirit of God." (1 Corinthians 2:14) He who like a brute animal seeks after sensual pleasures is not capable of even understanding the excellence of spiritual goods. In the soul which mortification reigns, all virtues shall flourish.

All our sanctity and salvation consist in following the examples of Jesus Christ. But we shall not be able to imitate Jesus Christ unless we deny ourselves, and embrace by mortification the cross that he gives us to carry. "If any man will come after Me, let him deny himself and take up his cross and follow Me." (Matthew 16:24)

Since Jesus Christ has endured so much for the love of us, it is but just that we suffer for his sake. We must, then, endeavor to follow the advice of St. Paul: "Always bearing about in your body the mortification of Jesus, that the life also of Jesus may be made manifest in our bodies." (2 Corinthians 4:10)

The principle means of acquiring sanctity are prayer and mortification, represented in the sacred Scripture by incense and myrrh. "Who is this that goeth up by the desert, as a pillar of smoke of aromatical spices, of myrrh and frankincense." (Canticles 3:6) St. Francis Borgia used to say, that prayer introduces divine love into the heart; but it is mortification that prepares a place for charity, by removing from the soul the world which should otherwise prevent the entrance of love. Should a person go to a fountain for water with a vessel full of earth, he shall take back nothing but mire. He must first cast away the earth, and then fill the vessel with water.

St. Ignatius of Loyola has said that the mortified soul unites herself more intimately to God in a quarter of an hour's prayer, than an immortified soul does in several hours.