Leo placed great confidence in the spiritual efficacy of fasting. "Although any time is suitable for restraint, this time is most fit because we see it as chosen by the appointment of the apostles and the laws, that, as in other days of the year, so in September, we should cleanse ourselves by spiritual purification" ( Serm. 93.3). The sermons for the four seasonal fasts have slightly different emphases. This group highlights the benefits of the corporateness of the act. Leo praises unity of worship and action as a protection against temptations. "A general fast ought to be celebrated on certain days by all together, for then devotion is more efficacious and more holy when . . . the understanding of the whole Church is one" ( Serm. 89.2), and "the people of God become more powerful when the hearts of all the faithful come together in the unity of holy obedience" ( Serm. 88.2). Nevertheless, with sensitivity, he realizes that each one's capacity is different and so "the infirm may be released from the labor that is beyond the powers of the body by the outlay of their resources" ( Serm. 87.3).
Serm. 89.1, given in 444, refers again to the Manichaeans and their customs, "the barefoot processions, the sad-faced fasts, the disreputable clothing," warning the audience and urging that "many fighting together against an enemy are in less danger than one fighting alone" ( Serm. 89.2).
Years later, in 453 ( Serm. 91.2), Leo is still fighting heresy. This time he condemns the Nestorians who damage the unity of Christ by "separating the human from the divine" in him, and the Eutychians who "obliterate the human in the divine." Faith and participation in the Blessed Eucharist provide a remedy against all errors ( Serm. 91.3).
WE KNOW your observance, dearly beloved, to be so devoted that you exercise your souls not only in the prescribed fasts, but in voluntary fasts as well. Still, to your eagerness we must add the encouragement of a reminder. In this way, if any are too irresolute in the practice of fasting, at