I myself did not know him, but he who sent me to baptize with water said to me, ‘He on whom you see the Spirit descend and remain, this is he who baptizes with the Holy Spirit.’ And I have seen and have borne witness that this is the Son of God.”(John 1:33, 34 ESV)Why did John say that he did not know Jesus? Surely he did. They were cousins. Surely he had heard about Jesus' miraculous birth, about his own calling, about leaping in his own mother's womb for joy at the presence of Jesus. Surely when Jesus came to get baptized (Matthew 3:16; Mark 1:10; Luke 3:22) and John felt unworthy to baptize him, but should have the roles reversed, surely he meant that because he knew who Jesus was... So why did John say he did not know him?
The answer is most likely that, even though John probably knew who Jesus was, the confirmation from God came through this sign that happened at Jesus' baptism. Seeing the Spirit descend on Jesus was what John had been waiting for so that he would know, beyond a shadow of a doubt, that Jesus was the Messiah!