12 “This is the bread which comes down from heaven.” Manna signified this bread; God's altar signified this bread. Those were sacraments. In the signs they were diverse; in the thing which was signified they were alike. Hear the apostle: “For I would not that you should be ignorant, brethren,” says he, “that all our fathers were under the cloud, and all passed through the sea; and were all baptized unto Moses in the cloud and in the sea; and did all eat the same spiritual meat.”
Of course, the same spiritual meat; for corporally it was another: since they ate manna, we eat another thing; but the spiritual was the same as that which we eat. But “our” fathers, not the fathers of those Jews; those to whom we are like, not those to whom they were like. Moreover he adds: “And did all drink the same spiritual drink.” They one kind of drink, we another, but only in the visible form, which, however, signified the same thing in its spiritual virtue. For how was it that they drank the “same drink”?
“They drank,” says he “of the spiritual Rock that followed them, and that Rock was Christ.” Thence the bread, thence the drink. The rock was Christ in sign; the real Christ is in the Word and in flesh. And how did they drink? The rock was smitten twice with a rod; the double smiting signified the two wooden beams of the cross. “This, then, is the bread that comes down from heaven, that if any man eat thereof, he shall not die.” But this is what belongs to the virtue of the sacrament, not to the visible sacrament; he that eats within, not without; who eats in his heart, not who presses with his teeth.
Source: Tractates on the Gospel of John (New Advent)