We know indeed, dearly beloved, your devotion to be so warm that in the fasting, which is the forerunner of the Lord’s Easter, many of you will have forestalled our exhortations. But because the right practice of abstinence is needful not only to the mortification of the flesh but also to the purification of the mind, we desire your observance to be so complete that, as you cut down the pleasures that belong to the lusts of the flesh, so you should banish the errors that proceed from the imaginations of the heart.
For he whose heart is polluted with no misbelief prepares himself with true and reasonable purification for the Paschal Feast, in which all the mysteries of our religion meet together. For, as the Apostle says, that “all that is not of faith is sin,” the fasting of those will be unprofitable and vain, whom the father of lying deceives with his delusions, and who are not fed by Christ’s true flesh.
As then we must with the whole heart obey the Divine commands and sound doctrine, so we must use all foresight in abstaining from wicked imaginations. For the mind then only keeps holy and spiritual fast when it rejects the food of error and the poison of falsehood, which our crafty and wily foe plies us with more treacherously now, when by the very return of the venerable Festival, the whole church generally is admonished to understand the mysteries of its salvation.
For he is the true confessor and worshipper of Christ’s resurrection, who is not confused about His passion, nor deceived about His bodily nativity. For some are so ashamed of the Gospel of the Cross of Christ, as to impudently nullify the punishment which He underwent for the world’s redemption, and have denied the very nature of true flesh in the Lord, not understanding how the impassible and unchangeable Deity of God’s Word could have so far condescended for man’s salvation, as by His power not to lose His own properties, and in His mercy to take on Him ours…
Pope Leo the Great continues at The Catholic Thing.