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. - John 3:18 (KJV)
There was a TV program called, "Lets Make A Deal", where contestants were shown numbered doors and told to pick one. Behind one door would be a great prize, behind another a donkey or some silly thing. Of course, the contestant had neither before the choice and would often hesitate, afraid of picking the wrong door.
Many, look at faith that way. They think they have choices, as in more than one. Choosing to believe in Christ would bring one result and choosing not to believe in Christ would bring another, and they wait, thinking that hesitation insulates them from bad (or unpleasant) consequences - Not so!
Jesus' admonition in John 3:18, is that there is really only one choice because not believing leaves one in the current condemned and fallen state, and that "condemnation" is a life lived in "Darkness" which just never ends.
And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil. For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.. - John 3:19-20 (KJV)
Being in the dark is a lonely place, even in the midst of a crowd; it puts one in proximity of a lot of unsavory creatures, who also live in the dark; and it places one under the "authority" of darkness (Colossians 1:13).
