Leo extols the benefits of fasting (prescribed by both the Old and the New Testaments) in a number of his sermons. In Serm. 19.2 he refers to the spring fast of Lent, the summer fast after Pentecost, the autumn fast in the seventh month (September) and the winter fast in the tenth month (December). Leo upholds the necessity of bodily and spiritual fasts, stressing that abstinence from food must be completed with prayer and almsgiving.
He also enumerates works of mercy which must accompany fasting: defending widows, taking care of orphans, consoling mourners, making peace between factions, welcoming travelers, relieving the oppressed, clothing the naked, tending to the sick ( Serms. 13.1(2) and 16.1). These public fasts derive a great beauty from the strength communicated to individuals when all pursue a common goal.
In Serm. 9 from the November Collections of 443, Leo asks his flock to reveal any hidden Manichaeans. Here, in Serm. 16.1 from the December Fast of 443, he warns his people to beware of heretics, naming Basilides, Sabellius, Photinus, Arius, and Eunomius. He mentions that Manichaeans had been investigated and brought to trial ( Serm. 16.4).
17 December 450
IF WE REFLECT upon the beginning of our creation with faith and wisdom, dearly beloved, we shall come to the realization that human beings have been formed according to the image of God precisely with a view that they might imitate their Designer. Our race has this dignity of nature, so long as the figure of divine goodness continues to be reflected in us as in a kind of mirror.
Indeed, the Savior's grace re-fashions us to this image on a daily basis. What fell in the first Adam has been raised up in the second. But our being re-fashioned has no other cause than the mercy of God. We would not love him but for the fact that