20 This was the figure which Moses completed by fixing the serpent to a cross, that whoso had been bitten by the living serpent, and looked to the brasen serpent, might be saved by believing. Does then the brazen serpent save when crucified, and shall not the Son of God incarnate save when crucified also? On each occasion life comes by means of wood. For in the time of Noe the preservation of life was by an ark of wood. In the time of Moses the sea, on beholding the emblematical rod, was abashed at him who smote it; is then Moses' rod mighty, and is the Cross of the Saviour powerless? But I pass by the greater part of the types, to keep within measure. The wood in Moses' case sweetened the water; and from the side of Jesus the water flowed upon the wood.
Source: Catechetical Lectures (New Advent)