I myself did not know him, but the reason I came baptising with water was that he might be revealed to Israel.”
I knew him not = we don’t know if John personally knew Jesus or not. With John’s secluded upbringing, this is a possibility (Luke 1:80). Before he was born, he apparently recognized the Messiah (Luke 1:41) and he was looking for Him during his ministry, but until John baptized Jesus, he did not know who the Christ was.
Luke 1:80 And the child grew and became strong in spirit; and he lived in the desert until he appeared publicly to Israel.
Luke 1:41 When Elizabeth heard Mary’s greeting, the baby leaped in her womb, and Elizabeth was filled with the Holy Spirit.
but the reason … = the major purpose of John’s baptism was not to bring men to repentance; it was an advertising campaign to show all of Israel who the Messiah was.