"If ye believe not that I am He, ye shall die in your sins" (John 8:24).
These are frightful words. St. John Chrysostom tells us (http://www.ccel.org/ccel/schaff/npnf114.iv.lv.html) that Christ came to take away the sin of the world, and that in baptism our sins are washed away and we "put off" the old man, our sinful nature.
Sin is a frightful thing, for it separates us from God. Christ came to free us from our sins and unite us to Himself. But if we choose again the old man, the life of sin, what then? St. John: "Wherefore He said, “He that believeth not is judged already”; not merely through his not believing, but because he departeth hence having his former sins upon him."