“It  is written (Jn 1:29): ‘Behold the Lamb of God, behold Him Who taketh  away the sin of the world’: and the reason for the employment of the  singular is that the ‘sin of the world’ is original sin, as a gloss  expounds this passage.”—St. Thomas Aquinas, Sum. Theo., Prima Secundae,  Q. 82, Art. 2