What does Mark 1:2 mean?

Although Mark was writing to Gentiles with little to no experience or interest in Judaism (Acts 15:22–29), he starts with John the Baptist who is considered the last of the Old Testament prophets. Many Gentiles had only heard of John's baptism of repentance and not the baptism of the Holy Spirit (Acts 19:1–6). So Mark begins his Gospel by quoting Old Testament passages which anticipated John's ministry. The phrase "before your face," used in the ESV, simply means John preceded Jesus; the NASB translates this as "I SEND MY MESSENGER AHEAD OF YOU."

John the Baptist's ministry prepares the way for Jesus' message in two ways. First, he reminds people of their sins. Second, he transitions their thinking from national salvation towards an individual salvation. The Old Testament prophets often spoke of the sins of the nation as a whole, and even godly prophets took on the guilt of the people they admonished (Isaiah 6:5). John speaks to individuals, encouraging them to acknowledge their personal sins and ask God for forgiveness.

The exact words quoted in Mark 1:2 are taken from Exodus 23:20 and Malachi 3:1, not Isaiah. Some later Latin manuscripts state "in the prophets" instead of "Isaiah the prophet." These manuscripts, however, are late and appear motivated to remove the concern in this passage of Mark referring to Isaiah when he clearly includes multiple prophets in his quotation. The words cited in Mark 1:3 are taken verbatim from Isaiah, and are the subject of the reference beginning this verse.

God rarely springs significant spiritual truths on us without any preparation. The Old Testament describes centuries of God warning His people not to sin, and dozens of prophecies calling the Israelites to look for the Messiah's coming. Similarly, He will often send messengers to us, preparing our hearts for His truth. Our responsibility is to listen and take God's warning.