"As John's disciples were leaving, Jesus began to speak to the crowd about John: 'What did you go out in the wilderness to see? A reed swayed by the wind? If not, what did you go out to see? A man dressed in fine clothes? No, those who wear fine clothes are in kings' palaces. Then what did you go out to see? A prophet? Yes, I tell you, and more than a prophet. This is the one about whom it is written: "I will send my messenger ahead of you, who will prepare your way before you."'" NIV translation
John's disciples left with the reassurance from the Scriptures that Jesus is the promised Messiah. After they left, we see that Jesus begins to ask the crowd about John the Baptist. We see that three times He asks them what they went out to see when John was baptizing in the wilderness and we are reminded of the fact that, throughout the Bible, the number three is associated with the display of God's will. Jesus tells them that John was not one that was swayed by public opinions like a piece of grass in the wind.
Jesus reminds them that John was not some rich and famous celebrity but that he was a messenger from God. He goes on to quote the words of the Father in heaven that were shared by the prophet Malachi concerning the fact that John had a special message (see Malachi 3:1)
In Jesus' description of John, we see that he was a humble messenger who simply went about sharing the message that God had entrusted to him.
This reminds us of the fact that God usually uses the humble and the lowly for His major missions. This type of person does not need to have all of the answers to do the work as they will simply submit to the leading of God. God knew that John would have these doubts as he sat in his jail cell but God also knew that John would not let those doubts stop him from following the instruction of the Lord.
For us, as Christians, it is a reminder that God will take us (doubts and all) and use us for His glory if we are willing to humbly act on the measure of faith that we have been given.