17 We often find in the symbolic passages of Scripture, that the same person appears in different characters on different occasions. So, on this occasion, Moses represents and prefigures the Jewish people as placed under the law. As, then, Moses, when he struck the rock with his rod, doubted the power of God, so the people who were under the law given by Moses, when they nailed Christ to the cross, did not believe Him to be the power of God. And as water flowed from the smitten rock for those that were thirsty, so life comes to believers from the stroke of the Lord's passion.
The testimony of the apostle is clear and decisive on this point, when he says, "This rock was Christ." In the command of God, that the death of the flesh of Moses should take place on the mountain, we see the divine appointment that the carnal doubt of the divinity of Christ should die on Christ's exaltation. As the rock is Christ, so is the mountain. The rock is the fortitude of His humiliation; the mountain the height of His exaltation. For as the apostle says, "This rock was Christ," so Christ Himself says, "A city set upon an hill cannot be hid," showing that He is the hill, and believers the city built upon the glory of His name.
The carnal mind lives when, like the smitten rock, the humiliation of Christ on the cross is despised. For Christ crucified is to the Jews a stumbling-block, and to the Greeks foolishness. And the carnal mind dies when, like the mountain-top, Christ is seen in His exaltation. "For to them that are called, both Jews and Greeks, Christ is the power of God, and the wisdom of God." Moses therefore ascended the mount, that in the death of the flesh he might be received by the living spirit.
If Faustus had ascended, he would not have uttered carnal objections from a dead mind. It was the carnal mind that made Peter dread the smiting of the rock, when, on the occasion of the Lord's foretelling His passion, he said, "Be it far from You, Lord; spare Yourself." And this sin too was severely rebuked, when the Lord replied, "Get behind me, Satan; you are an offense unto me: for you savor not the things which be of God, but those which be of men." And where did this carnal distrust die but in the glorification of Christ, as on a mountain height?
If it was alive when Peter timidly denied Christ, it was dead when he fearlessly preached Him. It was alive in Saul, when, in his aversion to the offense of the cross, he made havoc of the Christian faith, and where but on this mountain had it died, when Paul was able to say, "I live no longer, but Christ lives in me?"
Source: Reply to Faustus the Manichaean (New Advent)