We must repent for our sins, but knowing none of us would do this perfectly, He came also to do this for us, hence His baptism by John though John did not want to baptize his Lord, the Lord states that it must be He repent for us that the law be fulfilled, the law of repentance. If any of us thinks he has repented perfectly of every single sin we have committed in thought, word or deed - well dream on sinners! But Christ our Savior and Redeemer repented perfectly for each and every sin we have or ever will commit out of His grace and mercy for us. Hence His baptism by John makes perfect our imperfect repentance and we can trust that as we daily confess our sin and repent of these, receive His grace and forgiveness, that as woefully inadequate is our repentance, His righteous repentance for our sins cover us.