[“Who taketh away the sin of the world”.] He uses the word sin in the singular number, for any kind of iniquity; as if he had said, that every kind of unrighteousness which alienates men from God is taken away by Christ. And when he says, the sin OF THE WORLD, he extends this favour indiscriminately to the whole human race; that the Jews might not think that he had been sent to them alone. But hence we infer that the whole world is involved in the same condemnation; and that as all men without exception are guilty of unrighteousness before God, then need to be reconciled to Him. 

Commenting on John 1:29 - John Calvin

Let us note well the lesson which is here shown to all believers: namely, when we would wish to walk properly, we must not be as it were only before men, our eyes must not be focused only on them; but we must contemplate the heavenly Judge, and we must know that it is to Him that we have to answer and render account.
—  John Calvin