I think that what John was trying to say is that the people of the world used to sacrifice the lamb to cleanse their sins. However with so many sins that each of us do, there would not be any Lambs left. so Instead the Lord Almighty sent a Lamb of His Own,"Jesus". And through Jesus he would be sacrificed to cleanse all of the sins of the world from past to present. So John knew that Jesus would do his ministery of God's work and then later in Jesus' life, Jesus would be put to the cross and then be sacrificed, like a Lamb. Knowing that it was not the "lamb of the people", But the "Lamb of God", that it was God who would sacrifice His Lamb, not the people. And that the people must realize that only One sacrifice was made for all their sins if they would only believe in Him.