In this quote I say that I believe those who are condemned are condemned because they do not believe in Christ.
17 For God sent not his Son into the world to condemn the world; but that the world through him might be saved.
18 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.
Lets take a step back for a moment and consider verse 18 in its immediate context. Notice that in verse 17 that God did not send his son into the world to condemn the world but to that the world might be saved. Hence, that necessarily infers the world was already in a condemned state prior to God sending his Son into the world and in need of salvation not condemnation.
Second, notice the shift of plural and singular subjects between verses 17 and 18 and then again from verse 18 to verse 19 and then again in verse 21. In verse 17 the subject is "the world" whereas in verse 18 it is individualized "he" while in verse 19 it is changed again to generic "men" and then back again to the singular in verses 20-21.
Now, he shifts between God's design for sending his son into the world in verse 17 to the individualized response to the Son's presence among men in verse 18. Thus, the shift from "the world" in verse 17 to "he....him" in verse 18. This world is already condemned before the Son comes into the world but verse 18 deals with the individual response of Christ now present in the world and that is why Jesus uses PRESENT TENSE participles "beleiveth.....beleiveth" to describe the present response to Christ among men with regard to God's divine intent. He that is believing is no longer under condemnation, but he that is not believing remains under that condemnation. So, he is not saying that believing or not believing determines the condemned state as something other than that had already determined that before the Son came into the world. What is that?
That is the subject of verse 19 where again he shifts subjects from the singular "he....him" to the plural "men...their." Here he defines the original cause for condemnation that preceded the Christ coming into the world but was a condition existed when Christ came into the world. Unbelief is rooted in something far more sinister and that is "men" in general "loved" (previous to Christ's coming) darkness and "hateth" light. This is the CAUSE for their unbelief in him when he came into the world, this was their condition of condemnation that prevented them from receiving him in belief. Notice the completed action verbs "loved....were evil" which are evidence for their already condemned condition prior to the light coming into the world.
19 And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
It is this previous condition that manifests their "already condemned" condition and explains why they did not receive Christ when he came into the world. Now, Jesus changes again the subject from a plural to the singular "every one...his" in verses 20-21 where he provides explanation for why their present state of unbelief as "For" is explanatory -
20 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.
Additionally to their LOVE and HATE heart condition their PRESENT actions "DOETH EVIL" manifests their "already condemned condition" and it is this SIN CONDITION (state of the heart, kind of actions that issue from the heart) that is the cause for their present response in unbelief to Christ. Hence, Jesus is not saying that unbelief is the singular and only cause for their condemnation but that it is merely one more manifestation of a deeper reason for that condemnation.
21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.
Hence, the condition of their heart (hateth, loveth) is responsible for their inability to come to Christ in faith and is the cause for doing evil, thus THIS IS the underlying cause they are "condemned already." In direct, contrast "he that doeth" and "cometh to the light" is not due to him but a change of their basic love and hate nature which prohibits them from coming and such a change is the manifestation "they are wrought in God" or the "new birth" that Jesus introduced to Nicodemus at the beginning of this discussion.
Thus, Jesus has concluded his discussion with proof why the new birth is necessary as the root of unbelief is found in an evil nature that loves darkness and hates light and that is why men do not come to the light.