Why does Matthew explain the need for Jesus' water baptism? What is going on with this community of Matthew? What kind of theology does Matthew advocate?
There are at least two plausible concerns or questions raised by some members of the Matthean community:
1) Why does Jesus need the water baptism by John since his baptism is for the forgiveness of sins?
2) Is Jesus's authority lower than John's since he was baptized by John?
Jesus's explanation is simple. His baptism is necessary for fulfilling the righteousness (of God), which is a prevalent theme in Matthew, not to mention the Hebrew Bible. Baptism means a surrender to God, a new start for his public ministry, and his commitment to advocating the justice of God. Otherwise, his baptism is neither about the forgiveness of sins nor about a matter of authority or ranking.
The long conversational style of the Matthean baptism story is consistent with the didactical style of Matthew’s Gospel, composed of plenty of teaching materials such as the Sermon on the Mount and parables of Jesus.
When it comes to Luke, why does Luke emphasize people's presence at Jesus's baptism and prayer of Jesus? Indeed, the Gospel of Luke as a whole is imbued with this theme of the down-to-earth ministry perspective along with the importance of prayer. Why did Luke omit baptism details from Mark? It would be hard to know. Perhaps, one plausibility is that in Luke's mind, those details are considered not crucial to the gentile audiences.