John 8:24
“I said therefore unto you that...if you believe not that I am He, you shall die in your sins.”
For if He came to take away the sin of the world, and if it is impossible for men to put that off in any other way except by the washing, it needs must be that he that believes not must depart hence, having the old man; since he that will not by faith slay and bury that old man, shall die in him, and shall go away to that place to suffer the punishment of His former sins. Wherefore He said, “He that believes not is judged already”; not merely through his not believing, but because he de parts parts hence having his former sins upon him.
Source: Homilies on the Gospel of John (New Advent)