Calvin:

“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 favor 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, they need to be reconciled to him. John the Baptist, therefore, by speaking generally of the sin of the world, intended to impress upon us the conviction of our own misery, and to exhort us to seek the remedy. Now our duty is, to embrace the benefit which is offered to all, that each of us may be convinced that there is nothing to hinder him from obtaining reconciliation in Christ, provided that he comes to him by the guidance of faith. Besides, he lays down but one method of taking away sins.

Calvin, Commentary, John 1:29.