Eusebius of Caesarea

This, too, was fulfilled, was clearly fulfilled, by our Savior's miraculous works after John's preaching. Notice therefore how he bears good tidings to the desert, not generally or to any desert but to one particular desert by the bank of the Jordan. This was because John lived there and baptized there, as Scripture says, "John was in the desert baptizing." … I think the desert here is a symbol of that which of old was void of all God's good things, I mean the church of the Gentiles. And the river by the desert that cleanses all that are bathed therein is a figure of some cleansing spiritual power, of which the Scriptures speak, saying, "The movements of the river make glad the city of God." And this means the ever-flowing stream of the Holy Spirit welling from above and watering the city of God, which is the name for life according to God. This river of God, then, has reached even to the desert, that is the Gentile church, and even now supplies it with the living water that it bears.

Moreover, it is said in this prophecy that the glory of Lebanon and the honor of Carmel shall be given to this wilderness. What is the glory of Lebanon but the worship performed through the sacrifices of the Mosaic law, which God refused in the prophecy which says, "Why do you bring me Lebanon from Sheba? And of what service to me is the multitude of your sacrifices?" He has transferred the glory of Jerusalem to the desert of Jordan, since, from the times of John, the ritual of holiness began to be performed not at Jerusalem but in the desert. In like manner, too, the honor of the law and of its more external ordinances was transferred to the wilderness of Jordan for the same reason, namely, that they who need the healing of their souls no longer hastened to Jerusalem but to that which was called the wilderness, because there the forgiveness of sins was preached.