What does Mark 1:9 mean?

This verse transitions the focus of the narrative from John the Baptist to Jesus, and in Mark's typical fashion, it is done with no fanfare and little background. There is no mention of Jesus' birth, the Old Testament prophecies which His birth fulfilled, or even the timeframe, although this took place in the late 20s or early 30s AD. Jesus was born in Bethlehem (Luke 2:1–20), but was primarily raised in Nazareth of Galilee (Matthew 2:19–23). Matthew 3:13–17 and John 1:29–34 give more detail about Jesus' baptism by John.

Baptism, even in John's era, has been a sign that you believe and follow the words of the baptizer. John is very reluctant to baptize Jesus. John wishes to publicly follow Jesus and doesn't understand how he could baptize someone whose sandal he was not worthy to untie (Mark 1:7). But in being baptized by John, Jesus validates John's message of repentance and forgiveness. Where in Mark 1:5 people from all over the region confess and repent, here Jesus affirms their actions. Eventually, He will go further, taking on their sins at the cross so they can be fully forgiven and their relationship with God restored.

It is here that Jesus sets the precedence for our baptism. Although baptism is not necessary for salvation, it is a public statement that we repent of our sins and seek God's forgiveness. But Christian baptism goes beyond what John offered—it is an illustration of the baptism of the Holy Spirit and, therefore, a critical part of the instructions Jesus gave the disciples as they spread the gospel (Matthew 28:19–20). While John's baptism acknowledged our need for forgiveness, Christian baptism acknowledges that we are forgiven!