“what about the millions that lived and died before Christ was even born or who otherwise never had the opportunity to reject Jesus?”
A common answer is “we don’t know, but we do know about those, you for example, who have had that opportunity.”
But, we do know, if you read the Bible. There was a place where the righteous went - read the story of the rich man and Lazarus. When Jesus arose from the dead, the graves were opened and those who had died before Jesus’s resurrection came forth out of their graves. I believe that is in Matthew 24th chapter. So, yes, there was a holding place for those in the Old Testament.
Christ was promised in the garden at the time of the fall. God promised there would be a savior born of the seed of Eve. This promised was confirmed to Abraham. Those who looked forward and had faith in the promised savior were saved. Just as now those who look back and have faith in the revealed savior, Christ, are saved.