John had spent this life until that point preparing the way of the Lord. He had put his life on the line, proclaiming Jesus as "the lamb of God who takes away the sins of the world." Now John is in prison. Before he gives his life, he sends his disciples to ask Jesus if he is indeed the Christ. Why John's apparent doubt?
Being in prison gave John much time to think. As he considered his life he naturally wanted to be sure he was on the right path. I think he doubt arose from the simple fact that Jesus was his cousin. With family, you want to be sure!
Could Jesus, this man John had know since childhood, be the Messiah? Their first encounter came when Mary, now with child, travelled to see her cousin and John's mother Elizabeth, who was herself expecting after years of sterility. When Mary greeted Elizabeth, St. Luke tells us that John lept for joy while he was still in the womb.
Years later, when Joseph and Mary were returning from Jerusalem after celebrating Passover, they realized that Jesus was not with them. People travelled with extended family then. Luke tells us that "they were looking for him among their relatives and friends." It is certainly possible that John was in that group.
In any case, John was wondering about his cousin. Jesus' answer to John was to tell them "Go tell John what you hear and see: the blind regain their sight, the lame walk, lepers are cleansed, the deaf hear, the dead are raised and the poor have the Gospel preached to them." It is what Jesus does that proves who he is.
What about us. If someone were to ask if were could prove we were who we claimed to be, could we prove it? On a good day, maybe. What works could we point to and say this is what I have done; it shows who I am. It is good to be here for Mass to worship and grow in faith. Now, what about the rest of the week? It would be a shame if we were to receive Christ in faith in the Eucharist yet fail to become what we receive. We receive the Body of Christ. Let us become the body of Christ in the world.