St. John the Forerunner: Suffering for Truth Joyously

Orthodoxy proclaims that St. John the Forerunner suffered for the Truth joyously. So, what is this truth for which he suffered?

“If man in his religious life adopts the course of rational research, his approach to the world will inevitably be pantheistic. Every time the theologizing mind essays of its own strength to know the truth about God, whether or not it understands, fatally it falls into the same error in which science and philosophy and pantheism are sunk—intuiting truth as ‘WHAT’. Truth as ‘WHO’ is never arrived at through reason. God as ‘WHO’ can be known only through communion in being—that is, only by the Holy Spirit Staretz Silouan constantly emphasized this. The Lord spoke Himself of it thus:  ‘If a man love me, he will keep my words:  and my Father will love him, and we will come unto him, and make our abode with him … The Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things.’” (Archimandrite Sophrony, St. Silouan the Athonite, pg. 112)