John again recognizes and announces his recognition that Jesus is the Lamb of God.
Excursus: The Lamb of God is a term used in Revelation 17:14 describing the post-biblical apocalypse. Scriptural references include the servant songs like Isaiah 53:6-7 with vicarious suffering; Passover lamb, Exodus 12-13, not as a sacrifice for sin but reinterpreted in light of the eucharist (see 1 Corinthians 5:7-8).
John's disciples respond to his announcement by following Jesus. John knew who Jesus was because he had been told directly--and had bothered to listen (1:29-33). John does not keep this information to himself (34-36). His words and example are convincing to others (37).