"I didn't know he was the one, but when God sent me to baptize with water, he told me, 'The one on whom you see the Spirit descend and rest is the one who will baptize with the Holy Spirit.'"
Here we get some insight into John the Baptist's faith. When he first obeyed God and started baptizing with water, he didn't know whom he was preparing the way for. All he knew was that he was supposed to baptize with water and that God would point out "the one" in due time. He gave John the sign to look for. John looked and eventually saw. In the meantime, he trusted and obeyed God. That's faith