From: John 1:29-34
The Witness of John (Continuation)
 The next day he (John the Baptist) saw Jesus coming toward him, and said, "Behold, the Lamb of God, who takes away the sin of the world!  This is He of whom I said, `After me comes a man who ranks before me, for He was before me.'  I myself did not know Him; but for this I came baptizing with water, that He might be revealed to Israel."  And John bore witness, "I saw the Spirit descend as a dove from Heaven, and it remained on Him.  I myself did not know Him; but He who sent me to baptize with water said to me, `He on whom you see the Spirit descend and remain, this is He who baptizes with the Holy Spirit.'  And I have seen and borne witness that this is the Son of God."
29. For the first time in the Gospel Christ is called the "Lamb of God". Isaiah had compared the sufferings of the Servant of Yahweh, the Messiah, with the sacrifice of a lamb (cf. Isaiah 53:7); and the blood of the paschal lamb smeared on the door of houses had served to protect the firstborn of the Israelites in Egypt (cf. Exodus 12:6-7): all this was a promise and prefiguring of the true Lamb, Christ, the victim in the sacrifice of Calvary on behalf of all mankind. This is why St. Paul will say that "Christ, our Paschal Lamb, has been sacrificed" (1 Corinthians 5:7). The _expression "Lamb of God" also suggests the spotless innocence of the Redeemer (cf. 1 Peter 1:18-20; 1 John 3:5).
The sacred text says "the sin of the world", in the singular, to make it absolutely clear that every kind of sin is taken away: Christ came to free us from Original Sin, which in Adam affected all men, and from all personal sins.
The Book of Revelation reveals to us that Jesus is victorious and glorious in Heaven as the slain lamb (cf. Revelation 5:6-14), surrounded by saints, martyrs and virgins (Revelation 7:9, 14; 14:1-5), who render Him the praise and glory due Him as God (Revelation 7:10).
Since Holy Communion is a sharing in the sacrifice of Christ, priests say these words of the Baptist before administering it, to encourage the faithful to be grateful to our Lord for giving Himself up to death to save us and for giving Himself to us as nourishment for our souls.
30-31. John the Baptist here asserts Jesus' superiority by saying that He existed before him, even though He was born after him. Thereby he shows us the divinity of Christ, who was generated by the Father from all eternity and born of the Virgin Mary in time. It is as if the Baptist were saying: "Although I was born before Him, He is not limited by the ties of His birth; for although He is born of His mother in time, He was generated by His Father outside of time" (St. Gregory the Great, "In Evangelia Homiliae", VII).
By saying what he says in verse 31, the Precursor does not mean to deny his personal knowledge of Jesus (cf. Luke 1:36 and Matthew 3:14), but to make it plain that God revealed to him the moment when he should publicly proclaim Jesus as Messiah and Son of God, and that he also understood that his own mission as precursor had no other purpose than to bear witness to Jesus Christ.
32-34. To emphasize the divinity of Jesus Christ, the Evangelist includes here the Precursor's testimony regarding Jesus' Baptism (cf. the other Gospels, which describe in more detail what happened on this occasion: Matthew 3:13-17 and paragraph). It is one of the key points in our Lord's life, in which the mystery of the Blessed Trinity is revealed (cf. note on Matthew 3:16).
The dove is a symbol of the Holy Spirit, of whom it is said in Genesis 1:2 that He was moving over the face of the waters. Through this sign of the dove, the Isaiah prophecies (11:2-5: 42:1-2) are fulfilled which say that the Messiah will be full of the power of the Holy Spirit. The Baptist points to the great difference between the baptism he confers and Christ's Baptism; in John 3, Jesus will speak about this new Baptism in water and in the Spirit (cf. Acts 1:5; Titus 3:5).
"The Son of God": it should be pointed out that in the original text this _expression carries the definite article, which means that John the Baptist confesses before his listeners the supernatural and transcendent character of Christ's messiahship--very far removed from the politico-religious notion which Jewish leaders had forged.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.