The key to that verse (John 3:18) is "already condemned". Those that do not believe are ALREADY condemned because of sin. It isn't the unbelief that condemns us but that we stand condemned coming into the world due to our sin nature which is at enmity with God. Grace and faith is how we are saved from that by the blood of Christ cleansing us from all sin. I understand the point you are trying to make about those who we think never hear the gospel and never directly reject it and whether or not they are held accountable for what to us seems to be not their fault. In John 3:19, Jesus continues:
And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
We ALL are born into sin and on our way to eternal separation from God. But He steps in and shines the light of truth to the souls that seek Him and, as I said earlier, we cannot limit God in how He reveals Himself to the one who searches for Him with all their heart. Just look at Abraham.
It's like the Catholic mindset is that people are saved by default until they reject Christ which is not a Scriptural position, but rather the truth is, we are condemned until we accept Christ.(John 1:12-13)
The key here seems to be belief.
We are already condemned.
My point, and the point of the Church is that we don’t know what is in the heart of those who never hear of Jesus.
Here’s my thing. God first revealed Himself to the Jewish people and even though they were His Chosen People, they still did not have salvation.
In Jesus, we know that the covenant is no longer reserved just to the Jewish people but for all the world.
Why then would God allow eternal condemnation to people who have never even heard of Jesus, and therefore had no chance to believe or not believe?
Trusting in His limitless mercy, we also trust that He will make Himself known to all, even if we aren’t aware of it.
If they sincerely seek God, then I believe God will not turn away, because of two things, His love for His Son and His love for man.
If they sincerely seek God, we already know that God must be working in them as none of us would seek Him without His grace first.