29 Of the departure of Israel from Egypt, let us hear what the apostle himself says: "I would not, brethren, that you should be ignorant that all our fathers were under the cloud, and all passed through the sea, and were all baptized into Moses in the cloud and in the sea, and did all eat the same spiritual meat, and did all drink of the same spiritual drink. For they drank of the spiritual rock which followed them, and that rock was Christ." The explanation of one thing is a key to the rest.
For if the rock is Christ from its stability, is not the manna Christ, the living bread which came down from heaven, which gives spiritual life to those who truly feed on it? The Israelites died because they received the figure only in its carnal sense. The apostle, by calling it spiritual food, shows its reference to Christ, as the spiritual drink is explained by the words, "That rock was Christ," which explain the whole. Then is not the cloud and the pillar Christ, who by His uprightness and strength supports our feebleness; who shines by night and not by day, that they who see not may see, and that they who see may be made blind? In the clouds and the Red Sea there is the baptism consecrated by the blood of Christ. The enemies following behind perish, as past sins are put away.
Source: Reply to Faustus the Manichaean (New Advent)