Marah was a spring of bitter water. When Moses threw wood into it, its water became sweet. Water, you see, is of no avail for future salvation without the proclamation of the Lord's cross. But when it has been consecrated through the saving mystery of the cross, it is then ready for use in the laver of the Spirit and in the cup of salvation. Therefore, as Moses in his role of prophet threw wood into the spring of Marah, so also the priest sends out into the fountain of baptism the proclamation of the Lord's cross, and the water becomes sweet, ready for the giving of grace (On the Mysteries, 12-16; as cited in Wright, Readings for the Daily Office from the Early Church, p. 369, emphasis added).
11 September 2008
This picturesque explanation of Baptism and the Holy Supper comes from St. Ambrose, Bishop of Milan, in his treatise On the Mysteries.