The voice of one crying in the wilderness: Prepare the way of the Lord, make straight the paths of our God. The prophecy makes clear that it is to be fulfilled, not in Jerusalem but in the wilderness: it is there that the glory of the Lord is to appear, and God’s salvation is to be made known to all mankind. It was in the wilderness that God’s saving presence was proclaimed by John the Baptist, and there that God’s salvation was seen. The words of this prophecy were fulfilled when Christ and his glory were made manifest to all: after his baptism the heavens opened, and the Holy Spirit in the form of a dove rested on him, and the Father’s voice was heard, bearing witness to the Son: This is my beloved Son, listen to him.
A commentary on Isaiah by Eusebius of Caesarea