I. Since the Apostles' day till now self-restraint is the best defence against the Devil's assaults
Today's festival, dearly-beloved, hallowed by the descent of the Holy Ghost, is followed, as you know by a solemn fast, which being a salutary institution for the healing of soul and body, we must keep with devout observance. For when 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.
Source: Sermons (New Advent)