WmJas has clarified for me that the phrase that Jesus was 'the Lamb of God', was a reference to the Passover feats, and its use of a male lamb's blood to avert death.
Thus, when (in the Fourth Gospel) John the Baptist calls Jesus the Lamb of God that takes away the sin of the world, he was not using sin to mean immorality (as we do nowadays). JtB intended sin to mean (something like) the condition of mortality.
Sin is about mortality, not morality.
We need an explanation that fits the cure Jesus was offering - eternal resurrected life - with the disease from which Man is suffering, ie. sin.
Rephrased: The true explanation of John's phrase must make sense of 'sin' in such a way such that life everlasting could be understood to cure it.
Question: Why and how would Men being resurrected cure the world of sin?
Answer: When sin means our mortal state, and all that entails - corruption, decay, disease, weakness as well as death - sin is (and is caused by) the impermanence and transitoriness of all things.
That was the sin of the world which Jesus came to take-away.