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)
I like this verse, especially for what it does not say.
It does not say “He that believeth on Him will not be condemned as long as he holds on”.
If one has trusted in Jesus Christ (the “him” of the verse), one “is” (present state) not condemned. This is why people can stand on this verse and say that they know they are saved; this is not arrogance: it is merely standing upon what God’s word says.
The second part of this verse is also pretty solemn: it’s another “is” (present state). The one who does not believe already has a known outcome: he is “condemned already”. Not even a large pile of good works can change this state. (As an aside, I believe that the Bible infers that the degree of punishment suffered by a non-believer can be mitigated by the good works he does).