Much damage has been done to the cause of the gospel because of a wrong understanding of John 16:9. Here is the verse in context (speaking of the coming Holy Spirit): “And when he is come, he will reprove the world of sin, and of righteousness, and of judgment: of sin, because they believe not on me.”
It seems clear that the world will be judged because they didn’t believe in Yahushua. However, let me give a (somewhat lacking) analogy to bring another perspective.
If a man jumps out of a plane at 10,000 feet without a parachute he will perish because of the results of gravity. Had he trusted in a parachute, he would have been saved. However, the primary reason he perished was because of the result of the 10,000 foot fall. Again, the main reason he died was because of the fall, not because he failed to put on a parachute.
Sinners will be damned on Judgment Day because they violated God’s Law. The Scriptures give us a definition of “sin.” It is clear: “For sin is transgression of the Law” (1 John 3:4). If sinners had put on the Lord Yahushua Mashiach / Christ they would have been saved, but the primary reason men will be judged will be because they have violated God’s Law for sin is transgression of the Law.
Imagine that you are in the heart of darkest Africa, peering through the bushes at a very primitive tribe. They have never seen a missionary let alone heard the gospel. So they have never heard of Yahushua, and therefore never rejected Him. So if sin is “rejecting Yahushua,” they haven’t yet sinned. I say “yet” because the moment you preach Yahushua to them and they reject Him, they become sinners and will be going to Hell. So it’s best for them that you keep your mouth shut and just go home. And even when you get home you had better keep quiet about Yahushua because you will give more people opportunity to reject Him, and therefore they will be damned.
Here’s where the damage is being done by this incorrect interpretation. I have heard sinners say, “I’ve been told that I’m going to Hell just because I don’t believe in Yahushua.” To them it is understandably unfair (and even ludicrous) that anyone could believe that a just God would damn people to everlasting Hell, just because they didn’t “believe” something.
How much more reasonable is it to open up the divine Law as Yahushua did, and show sinners that God considers lust to be adultery and hatred to be murder, and because of His perfect holiness that His judgment is fair and just. That is reasonable. It makes sense. It is when they have an understanding of sin (which comes by the Law) that sinners can then see their need to trust in Yahushua as their sin bearer and be clothed in His perfect righteousness.