The next day again John [the Baptist] was standing with two of his disciples, and he looked at Jesus as He walked by and said, “Behold, the Lamb of God!” The two disciples heard him say this, and they followed Jesus … One of the two who heard John speak and followed Jesus was Andrew, Simon Peter’s brother. He first found his own brother Simon and said to him, “We have found the Messiah” (which means Christ). He brought him to Jesus. John 1:35-36, 40-42a ESV1
Read John 1:11-51
Andrew came to faith in Jesus by the testimony of John the Baptist. Andrew had been a disciple, or a follower, of John. As is the nature of being a disciple, Andrew had likely spent a great deal of time with and listening to John. And John spent much time talking about the coming Messiah. So much so that even casual observers of John were able to say, “Everything he [John] said about this man has come true” (John 10:41). Because of his close association with John and John’s teachings, Andrew was ready to follow the Messiah as soon as John identified him. “‘Behold, the Lamb of God!’ The two disciples heard him say this, and they followed Jesus” (John 1:36b-37).