In enjoining his hearers to maintain the disciple of regular fasting, St. Leo begins his sermon with these remarkable sentences.
[W]hen the Apostles had been filled with the promised power, and the Spirit of Truth had entered their hearts, we doubt not that among the other mysteries of heavenly doctrine this discipline of spiritual self-restraint was first thought of at the prompting of the Paraclete in order that minds sanctified by fasting might be fitter for the chrism to be bestowed on them. The disciples of Christ had the protection of the Almighty aid, and the chiefs of the infant Church were guarded by the whole Godhead of the Father and the Son through the presence of the Holy Ghost. But against the threatened attacks of persecutors, against the terrifying shouts of the ungodly, they could not fight with bodily strength or pampered flesh, since that which delights the outer does most harm to the inner man, and the more one's fleshly substance is kept in subjection, the more purified is the reasoning soul. And so those teachers, who have instructed all the Church's sons by their examples and their traditions, began the rudiments of the Christian warfare with holy fasts, that, having to fight against spiritual wickednesses, they might take the armour of abstinence, wherewith to slay the incentives to vice.And then, just before his concluding paragraph, St. Leo offers this exhortation to his hearers:
Therefore, after the days of holy gladness, which we have devoted to the honour of the Lord rising from the dead and then ascending into heaven, and after receiving the gift of the Holy Ghost, a fast is ordained as a wholesome and needful practice, so that, if perchance through neglect or disorder even amid the joys of the festival any undue licence has broken out, it may be corrected by the remedy of strict abstinence, which must be the more scrupulously carried out in order that what was on this day Divinely bestowed on the Church may abide in us. For being made the Temple of the Holy Ghost, and watered with a greater supply than ever of the Divine Stream, we ought not to be conquered by any lusts nor held in possession by any vices in order that the habitation of Divine power may be stained with no pollution.