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,
Read Chapter 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.
A tree, (lignum,) or piece of wood, which had the natural property here ascribed to it, Ecclesiasticus xxxviii. 4. (Calmet)
Though we can hardly suppose, that all that collection of waters would be thus rendered sweet, unless God had given it a miraculous efficacy. (Haydock)
It foreshewed the virtue of the cross. (Theodoret, ix. 26.)
Him, Moses, and the people of Israel, of which he was now the sole head or king. (Haydock)
God proved on this occasion the disposition of the Hebrews to enter into the alliance, of which he proposes to them the heads, ver. seq. Josue xxiv. 25, makes use of nearly the same words. God begins to take upon himself the administration of the republic, appointing the forms of judicature, Jeremias vii. 22. What regarded sacrifices, was given upon occasion of their idolatry. (Du Hamel)
In this mystical number, I say, the children of Israel, arriving at Marah and being unable to draw the water because of its bitterness (for the well had water but no sweetness, and it was pleasing to the eye but polluted to the taste), drank water that became sweet and mild as soon as wood was thrown into it by Moses. The sacrament of the wood removed the harshness that the noxious water bore. I believe that this happened as a sign, for I think that the bitter water of Marah is the Old Testament law, which was harsh before it was tempered by the Lord’s cross.
Again, water is restored from its defect to its native grace of “sweetness” by the tree of Moses. That tree was Christ, restoring of himself the veins of what had been envenomed and bitter nature into the allsalutary waters of baptism.