This is what I understand from this Brother Eli's video:
The baptism of Jesus is a part of the fulfillment of what was written. In Mt. 3:13-15, John the Baptist thought it was not right for Jesus to be baptized by him, but Jesus made it clear to him that it was right for him to fulfill all righteousness.
Jesus came to fulfill all righteousness. This is made clearer in Heb. 10:7 where it is written that he came to do the will of God as it written. In Lk. 24:44 Jesus said all things had to be fulfilled that was written about him in the law of Moses, and the Prophets and the Psalms.
While John was reluctant to baptize him, Jesus told him to go ahead to baptize him because there was need for us all to fulfill all righteousness.
The statement of Isaiah 40:3 where a voice was heard in the wilderness saying, prepare ye the way of the Lord was fulfilled by John who was the forerunner of Jesus.
Jesus was baptized, not because he had sinned, but simply to fulfill all righteousness.
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