How do you know that John the Baptist's statement about the lamb of God refers to sacrifice?
Throughout biblical times and even now humankind conforms God to its own cultural ideas, activities, and motives (whether good or bad) to gain understanding of Him...hence the many different religions. Every culture with awareness of a higher power shapes and molds God's character, laws, and abilities to fulfill their desire of closeness and oneness with Him. John's declaration of Jesus being "the lamb of God" who "takes away the sin of the world" in that era and setting was a statement that was made to provide understanding to those people at that time. I know that this statement refers to sacrifice because of the people it was being delivered to and the time it was delivered.
How was the comprehensiveness of sins of the world so radical a concept?
Throughout Hebrew scripture the Jews were promised a deliverer of their nation. They looked forward to, longed for, and prayed that the deliverer would come to them freeing their nation from oppression. They were promised a mighty King. So, when John said that the lamb would take away the sins "of the world" this broke away from the traditional belief that the deliverer would free only the Hebrew nation. Jesus came to deliver not only the Hebrews but also the Gentiles. I believe many Jews probably may have felt shocked and even cheated that Jesus was not their own personal deliverer but came to offer all of their promises to even their enemies, opressors, and slave masters. (holy smokes I know I would)