The big moment arrives and Jesus shows up at the Jordon River where John is baptizing. After all that preaching about repentance, John had been preparing his own heart for the moment when he would submit himself to be baptized by Jesus. Imagine his surprise when Jesus insists on being baptized by him. John feels a kind of horrified revulsion at the idea. He had just been saying earlier that he was not worthy to remove the sandals of Jesus' feet. A sinful man such as himself is certainly not worthy to baptize this one who has no need of repentance.
John's protest illustrates perfectly what we had discussed in the previous post about his mistaken expectation regarding Jesus' coming. John thought that Jesus was coming to bring final judgment to the world. What John didn't realize was that Jesus came not to judge, but to undergo judgment himself on the cross. That is why he tells John, "Permit it at this time, for in this way it is fitting for us to fulfill all righteousness."
A side note about baptism: I know that most of us are told that we are baptized "out of obedience to God's commandment." While that's true, there's a reason God commands it. Baptism symbolizes judgment. Colossians 2:12 says we have been "buried with [Christ] in baptism" and Romans 6:4 says "we have been buried with [Christ] through baptism into death." To undergo death and burial is to undergo judgment for your sins. When we're baptized in Christ we are symbolically undergoing judgment in Christ, who bears us through the judgment waters of death to the safe shores of resurrection and life by paying for our sins himself.
Getting back to John the Baptist, you can see why he is shocked that Jesus would asked to be baptized--as if he had sins for which he needed to be judged! But it isn't for his own sins that Jesus undergoes baptism, it is for ours. He has to identify himself fully with sinners in order to satisfy the demands of justice against us; that is why he tells John to "permit it at this time, for in this way it is fitting for us to fulfill all righteousness." When Jesus comes out from the water the voice of the Father and the descent of the Holy Spirit signal the beginning of his ministry, when he will begin to fulfill all righteousness on our behalf.