To the pure, all things are pure, but to those who are corrupted and do not believe, nothing is pure. In fact, both their minds and consciences are corrupted.
(Titus 1:15, New International Version)
Rust stains, grass stains, blood stains, chocolate stains -- ARGH! Those stains are real nightmares in the laundry room. They don't just leave a stain on the surface of the fabric, but they also leave a residue that infiltrates the fibers of our clothes. Evil is that way. We must not kid ourselves. Evil corrupts and leaves a residue that corrupts everything it touches, including the hearts of otherwise decent people. That's why we are urged to stay far away from evil and avoid its very appearance. That's also why Jesus died: to not only overcome the Evil One, but to also clean away the stain and its residue.