There is a difference between doubt and disbelief. Disbelief rejects Truth in spite of evidence, whereas doubt remains unsure, waiting further evidence for confirmation.
Sometimes our circumstances cause us to doubt, to question what we believe. Doubt is not denial - doubt holds disbelief at bay until further evidence is given to produce belief.
In our text, John's circumstances caused doubt. He has heralded the coming of the Messiah. His message has touched many hearts and the followers of Christ have increased. He has taken a stand against wickedness in high places, declaring that the marriage of Herod and Herodias was unlawful (Mark 6:18), thus incurring her wrath. Now, after doing God's will, he finds himself in prison, hardly a fitting end to an obedient servant of God and forerunner of the Messiah. No doubt he begins to wonder whether Jesus really is the Anointed One who was to come, or if he was just another of many false messiahs. He asked for assurance (vs 20).
Jesus' answer to John's messengers comes after He gave ample evidence (vs 21)...then He said, "Go tell John..."(vs 22). The evidence was that Jesus did (1) what no others had done, and (2) fulfilled prophecies about the Messiah.
John knew the Scriptures...John had witnessed to the fact that he believed that Jesus was the Messiah in declaring "Behold, the Lamb of God...". He just needed assurance at a time when his faith was at low ebb. He found that assurance in the witness of friends who returned with Jesus' message and with what they had seen.
Negative circumstances cause us to doubt, and we need affirmation of our faith, we need assurance. Sometimes, we just need a friend to come along side with words of affirmation and encouragement.
Why not be such a friend to someone today?
Joseph Scriven wrote this hymn our of a tragic event that occurred in his life. The day before he was to marry, his bride-to-be drowned. In an attempt to start life anew, he immigrated from Ireland to Canada. Some time later, he was again engaged to be married, but his fiance' died of pneumonia shortly before the wedding. Scriven wrote to his mother in Ireland, relating the tragic story and sending a copy of the poem "A Friend Who Understands", which he had written in 1857.
Years passed and his poem appeared titled "What a Friend", with music added by Charles Converse in 1868. It is assumed that his mother sent it to some Christian periodical where Converse came across it. In 1880 a neighbor of Scriven found the original manuscript and due credit was given to the author.
Perhaps you could use it to be a blessing and encouragement to someone today who needs words of affirmation in their faith.