The next day, he saw Jesus coming to him, and said, "Behold, the Lamb of God, who takes away the sin of the world!
The previous verses of this chapter give the historical situation leading to the call of the first disciples, one of the key elements in that situation having been the delegation from the religious community in Jerusalem and the ensuing discussion with John the Baptist, which resulted in a clearer definition of the true status of John as the harbinger of the Messiah and the forerunner of one even greater than himself.  The humble acceptance of the great herald of such a secondary and subordinate position was exceedingly significant in the eyes of his more perceptive disciples, especially to the deeply spiritual author of this Gospel; and their consequent awakening to the expectation of the Greater One paved the way for all that followed.