John the Baptist appears in all four Gospels — Matthew 3, Mark 1, Luke 3, and John 1 — as the one who preached in the wilderness and baptized Jesus in the Jordan.
Key passages:
"In those days came John the Baptist, preaching in the wilderness of Judaea," — Matthew 3:1
"For this is he that was spoken of by the prophet Esaias, saying, The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight." — Matthew 3:3
"And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:" — Matthew 3:16
"The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world." — John 1:29
Read the full accounts in Matthew 3, Mark 1, Luke 3, and John 1.