I’m afraid that all the responses so far have failed to understand John’s character. Christ himself states that among those born of women there has risen no one greater than John the Baptist.
You must ask yourself why did John still have disciples? Many of John’s disciples had already left him to follow Jesus and that is exactly as it should have been. As John said, “He must increase, I must decrease”
So here is John in prison, nearing the end of his life, the Christ has come and yet he still has disciples following him instead of Jesus. John knows that they need to follow Jesus instead of him but he also knows that they have to choose to follow Jesus of their own free will. If he ordered them to leave and follow Jesus they would still in a sense be following him as it would be out of obedience to John’s command.
So how does he deal with them? He tells his disciples to go and ask Jesus if He is the one who is to come or should they look for another? How does Jesus respond?
“Go and tell John the things which you hear and see: The blind see and the lame walk; the lepers are cleansed and the deaf hear; the dead are raised up and the poor have the gospel preached to them.”
Jesus, like John, leaves the choice to follow Him completely up to them. They were obviously fiercely loyal to John, probably zealous, and if you have had any dealings with people in the church who are zealously wrong about something, you know that they have to be treated with great care, otherwise they simply dig their heels in and refuse to budge on their position. John’s disciples needed careful treatment to be able to decide on their own that they needed to let go of John and follow Him whose way John had come to prepare.
John had no doubts at all that Jesus was indeed the Christ. He was a prophet after all.