"Behold, the Lamb of God, who takes away the sin of the world!” (v29). John the Baptist (not the John who wrote the gospel) was chosen by God to introduce Jesus to the world. People recognized John as an Old Testament-style prophet, whose unusual birth was also foretold by an angel (Luke 1), so they listened to him “reveal” or explain Jesus to others (v31). John described Jesus as the Lamb of God, the sacrifice that covers the sins not only of one person, or of Israel, but “the sin of the world.”
“Behold, the Lamb of God!” (v36). The Word who existed with the Father “in the beginning” became flesh and sacrificed himself to save us from our sins.