9 All things that were spoken to the ancient people Israel in the manifold Scripture of the holy law, what things they did, whether in sacrifices, or in priestly offices, or in feast-days, and, in a word, in what things soever they worshipped God, what things soever were spoken to and given them in precept, were shadows of things to come. Of what things to come? Things which find their fulfillment in Christ. Whence the apostle says, “For all the promises of God are in Him yea;” that is, they are fulfilled in Him.
Again he says in another place, “All happened to them in a figure; but they were written for our sakes, upon whom the end of the ages has come.” And he said elsewhere, “For Christ is the end of the law;” likewise in another place, “Let no man judge you in meat, or in drink, or in respect of an holy day, or of a new moon, or of Sabbath-days, which is a shadow of things to come.” If, therefore, all these things were shadows of things to come, also the feast of tabernacles was a shadow of things to come.
Let us examine, then, of what thing to come was this feast-day a shadow. I have explained what this feast of tabernacles was: it was a celebration of taber nacles, because the people, after their deliverance from Egypt, while directing their course through the wilderness to the land of promise, dwelt in tents. Let us observe what it is, and we shall be that thing; we, I say, who are members of Christ, if such we are; but we are, He having made us worthy, not we having earned it for ourselves.
Let us then consider ourselves, brethren: we have been led out of Egypt, where we were slaves to the devil as to Pharaoh; where we applied ourselves to works of clay, engaged in earthly desires, and where we toiled exceedingly. And to us, while laboring, as it were, at the bricks, Christ cried aloud, “Come unto me, all you that labor and are heavy laden.” Thence we were led out by baptism as through the Red Sea—red because consecrated by the blood of Christ. All our enemies that pursued us being dead, that is, all our sins being blotted out, we have been brought over to the other side.
At the present time, then, before we come to the land of promise, namely, the eternal kingdom, we are in the wilderness in tabernacles. They who acknowledge these things are in tabernacles; for it was to be that some would acknowledge this. For that man, who understands that he is a sojourner in this world, is in tabernacles. That man understands that he is travelling in a foreign country, when he sees himself sighing for his native land. But while the body of Christ is in tabernacles, Christ is in tabernacles; but at that time He was so, not evidently but secretly.
For as yet the shadow obscured the light; when the light came, the shadow was removed. Christ was in secret: He was in the feast of tabernacles, but there hidden. At the present time, when these things are already made manifest, we acknowledge that we are journeying in the wilderness: for if we know it, we are in the wilderness. What is it to be in the wilderness? In the desert waste. Why in the desert waste? Because in this world, where we thirst in a way in which is no water.
But yet, let us thirst that we may be filled. For, “Blessed are they that hunger and thirst after righteousness, for they shall be filled.” And our thirst is quenched from the rock in the wilderness: for “the Rock was Christ,” and it was smitten with a rod that the water might flow. But that it might flow, the rock was smitten twice: because there are two beams of the cross. All these things, then, which were done in a figure, are made manifest to us. And it is not without meaning that it was said of the Lord, “He went up to the feast-day, but not openly, but as it were in secret.” For Himself in secret was the thing prefigured, because Christ was hid in that same festal-day; for that very festal-day signified Christ's members that were to sojourn in a foreign land.
Source: Tractates on the Gospel of John (New Advent)