Matthew 11:2-3
And when John had heard in prison about the works of Christ, he sent two of his disciples and said to Him, “Are You the Coming One, or do we look for another?”

It’s difficult to imagine John the baptizer doubting the authenticity of Jesus.
It was John that declared, “Behold the Lamb of God that takes away the sins of the world!”
Everyone may doubt but Jesus always increases our faith, as He did for John.
But I go back and recall something John once said – “He must increase and I must decrease!”
I think perhaps John’s question wasn’t so much for himself but more for his disciples.
They need to learn to quit looking at John and being disappointed that he is in prison; rather look to Jesus as Messiah.
It was not John that witnessed the miracles of Jesus, it was his disciples that saw the blind see, the lame walk, lepers healed, the deaf get their hearing, and the dead are raised.
More importantly, they saw the Gospel preached to the poor.
Jesus is the Coming One and He is coming again!
Keep the faith!