Non-Jews had been being baptized to become Jews. John the Baptist had been baptizing people to show their repentance of sin. Why was Jesus baptized? He was born a Jew and not a sinner so Why? We can see several reasons in the Gospel Accounts:
1. Jesus was not baptized as a repentant sinner but was baptized to identify with the sinful people He came to save. God in flesh wanted to Identify with you! Remember, Jesus 'knew No sin'. He was sinless. He was tempted just as we are but did not fall to temptation. As a matter of fact, the very next account recorded in scripture is of His temptation by Satan himself. This baptism, this announcement of His identity would be very important to have and know during such a temptation.
2. It also pictured His future Baptism on the Cross (Matt 20:22 & Luke 12:50) when all of God's Judgment for all of man's sin would go on Him.
3. Matthew's Account reads - 'Then Jesus went from Galilee to the Jordan River to be baptized by John. But John tried to talk him out of it. “I am the one who needs to be baptized by you,” he said, “so why are you coming to me?” But Jesus said, “It should be done, for we must carry out all that God requires. ” So John agreed to baptize him.' Remember that Baptism (for the follower of Jesus) is the picture of the death, burial and resurrection of Jesus. We see the picture of The Gospel from the beginning of Jesus' adult ministry on earth.