He was also in a position to know what was involved in being the messiah.
"The Expositor's Bible: The Gospel of St. John, Vol. I"
Marcus Dods
No incident could more distinctly prove that the claim to be the messiah was in their judgment one thing, the claim to be Divine another thing.
"The Expositor's Bible: The Gospel of St. John, Vol. I"
Marcus Dods
With the exception of the mother of Jesus, John the Baptist appears to have been the only person, who, at this particular time, was perfectly convinced, without a word from Jesus Himself, that the long-expected messiah had appeared.
"The Making of an Apostle"
R. J. Campbell