Theophylact of Ohrid — as excerpted in the Catena Aurea on John 1:29-31

“(in loc.) He is called the Lamb of God, because God the Father accepted His death for our salvation, or, in other words, because He delivered Him up to death for our sakes. For just as we say, This is the offering of such a man, meaning the offering made by him; in the same sense Christ is called the Lamb of God Who gave His Son to die for our salvation. And whereas that typical lamb did not take away any man’s sin, this one hath taken away the sin of the whole world, rescuing it from the danger it was in from the wrath of God. Behold Him1 Who taketh away the sin of the world: he saith not, who will take, but, Who taketh away the sin of the world; as if He were always doing this. For He did not then only take it away when He suffered, but from that time to the present, He taketh it away; not by being always crucified, for He made one sacrifice for sins, but by ever washing it by means of that sacrifice.”