Jesus said "I am the way and the truth and the life. No one comes to the Father except through me"

There are many other places in the Bible that explain this as well. For example:

"You should have no other gods before me"

"Before me no god was formed, nor will there be one after me."

"For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. "

"to be saved through the sanctifying work of the Spirit and through belief in the truth"

As a Christian, it is clear from the Bible that our job is not to judge and condemn, that is up to God. Our job is to spread the good news. Their job is to accept or reject that good news.

It is clear that those who reject the Gospel will be subject to God's judment.
For those who have yet to hear the Gospel, it is unclear to me what happens to them. Clearly they haven't rejected the Word of God. This is why most Christian's believe babies, young children, and those with mental disabilties are probably saved or avoid judgement - because they haven't\can't be given the chance to choose. So to the question of 'what if...' - well that's up to God and the individual. But He makes it clear that my job is to spread the Gospel as the way for Salvation. Once you have the Truth, your free will kicks in and you choose or lose