Q1. How do you know that John the Baptist's statement about the Lamb of God refers to sacrifice? (John 1:29).
For John to call Jesus God's Lamb who would take away the sins of the world he had to know from God that in his (John's) mission as forerunner he was to proclaim the messiah's purpose - that of savior! but how much he understood is always a real point of question to me. Remember John sending his own disciples to Jesus and asking if He really was the one they looked for? I really think John like the writers of scripture was speaking God's words! The wonder to me is that in a real sense John is an old testament prophet who died without the cross happening! he may very well be included in the reference in 1Peter 1:11-12 as one who searched to know God's great gracious plan!
How was the comprehensiveness of "sins of the world" so radical a concept?
This is still something only God really understands!! but as others have said to the Jews this would be hard to hear with understanding! All the individual parts - a lamb's blood for sacrifice, God requiring a sacrifice for sin, all the people of the world being sinners, the personal need for atonement... all these things because of the Old Covenant the Jews would understand better than we do but it is only now that as Paul says that God's secret is explained, a people of God from both Jews and Gentiles!! Thanks be to God!!