He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.

Believing in him means you won't be judged negatively, but if you don't believe, you're already being judged - this is because you haven't trusted in the unique Son of God.

King James Version

Simple English

* Work in progress - there may be occasional inaccuracies