Marah was a fountain of most bitter water: Moses cast wood into it and it became sweet. For water without the preaching of the Cross of the Lord is of no avail for future salvation, but, after it has been consecrated by the mystery of the saving cross, it is made suitable for the use of the spiritual laver and of the cup of salvation. As, then, Moses, that is, the prophet, cast wood into that fountain, so, too, the priest utters over this font the proclamation of the Lord's cross, and the water is made sweet for the purpose of grace.
--Saint Ambrose, on the mysteries, c. 387
