Everyone knew that Jesus was the son of Joseph the carpenter, and here was John referring to Him as the Lamb of God. Can you imagine how Jesus must have felt every time a lamb was sacrificed for people's sins? He knew that one day He would be that sacrificial lamb, led to the slaughter to die for the sins of the world. Even though He knew this, He continued on and freely gave His life so that you and I might be saved. The spotless Son of God who had never showed anything but love, was despised and rejected.
We have so much to be thankful for. When we realize that all we have to do today, is call upon Jesus, God's spotless lamb. When we do, He will forgive us and come into our hearts. Praise God for the first time John proclaimed. "Behold the Lamb of God!"