And he cried unto the LORD; and the LORD showed him a tree, which when he had cast it into the waters, the waters were made sweet: there he made for them a statute and an ordinance, and there he proved them,
All Commentaries on Exodus 15:25 Go To Exodus 15
Ambrose of Milan
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.