30 The Israelites are led through the wilderness, as those who are baptized are in the wilderness while on the way to the promised land, hoping and patiently waiting for that which they see not. In the wilderness are severe trials, lest they should in heart return to Egypt. Still Christ does not leave them; the pillar does not go away. The bitter waters are sweetened by wood, as hostile people become friendly by learning to honor the cross of Christ. The twelve fountains watering the seventy palm trees are a figure of apostolic grace watering the nations.
As seven is multiplied by ten, so the decalogue is fulfilled in the sevenfold operation of the Spirit. The enemy attempting to stop them in their way is overcome by Moses stretching out his hands in the figure of the cross. The deadly bites of serpents are healed by the brazen serpent, which was lifted up that they might look at it. The Lord Himself gives the explanation of this: "As Moses lifted up the serpent in the wilderness, so must the Son of man be lifted up, that whosoever believes in Him may not perish, but have everlasting life." So in many other things we may find a protest against the obstinacy of unbelieving hearts.
In the passover a lamb is killed, representing Christ, of whom it is said in the Gospel, "Behold the Lamb of God, who takes away the sin of the world!" In the passover the bones of the lamb were not to be broken; and on the cross the bones of the Lord were not broken. The evangelist, in reference to this, quotes the words, "A bone of Him shall not be broken." The posts were marked with blood to keep away destruction, as people are marked on their foreheads with the sign of the Lord's passion for their salvation.
The law was given on the fiftieth day after the passover; so the Holy Spirit came on the fiftieth day after the passion of the Lord. The law is said to have been written with the finger of God; and the Lord says of the Holy Spirit, "With the finger of God I cast out devils." Such are the Scriptures in which Faustus, after shutting his eyes, declares that he can see no prediction of Christ. But we need not wonder that he should have eyes to read and yet no heart to understand, since, instead of knocking in devout faith at the door of the heavenly secret, he dares to act in profane hostility.
So let it be, for so it ought to be. Let the gate of salvation be shut to the proud. The meek, to whom God teaches His ways, will find all these things in the Scriptures, and those things which he does not see he will believe from what he sees.
Source: Reply to Faustus the Manichaean (New Advent)