What does Mark 1:3 mean?

Mark 1:2 describes this short section as a quote from Isaiah the prophet. Although the first half was actually from Malachi, Mark 1:3 is cited directly from Isaiah 40:3. This particular statement, from Mark, is the best-known portion of his quotation and is the main argument tying John the Baptist to the messenger prophesied in the Old Testament. This is why Mark refers to this as a comment from Isaiah.

The root word translated "cry," boao, can mean a shout of joy or pain, a call for help, or just the words of a strong voice, all of which may apply here. Unlike Jesus, John voluntarily taught in the wilderness around the Jordan River, away from the established synagogues and the overly strict Jewish leaders.

The passage from Isaiah quoted here is a promise of comfort for God's people. Isaiah 40:2 says "Speak tenderly to Jerusalem, and cry to her that her warfare is ended, that her iniquity is pardoned, that she has received from the LORD's hand double for all her sins." It's a promise that Israel's relationship with God will be restored and their sins forgiven. Isaiah 40:3–4 goes on to say the "voice" in the wilderness will "make straight in the desert a highway for our God." This same prophetic passages promises that valleys and hills will be evened out so that God and His people can more easily be together.

John literally embodies Isaiah 40:1–4 in that he is the voice (a prophet), crying out (preaching), in the wilderness (the countryside of Judea), laying the groundwork for Jesus' coming ministry. This is the same wilderness where Jesus would be tempted for forty days in preparation for His own public ministry (Mark 1:12–13). John's message to the people to turn away from their sins is the spiritual truth behind the geographic metaphor. His call to repentance spiritually prepared the Jews for the coming Messiah. In the same way, by rejecting our sins and seeking God's forgiveness, we smooth the way for our hearts to receive God and the forgiveness we so desperately need.