God existed long before religion.
Therefore, millions of people never had the chance to be Christians - and why would God play favourites with a handful of 'believers' giving them a privileged place in Heaven?
If He created humans then he loves them all, without exception.
As others have said, the ideas of Heaven and Hell come from a very early 'punishment and reward' system to keep people in line so they would obey liturgical laws. The church fined them if they did not so it was profit-driven too
Surely we no longer believe that the wonderfully altruistic Muslim, Jewish and other non-Christian workers who give their lives in the service of others and their God, are condemned to hell?
Have you read any books by Bishop John Shelby Spong? He struggled with this too and I found his words extremely enlightening and comforting.
Did you know there's a theory that travellers in the ancient world saw the lands of Africa with its unbearable heat, savage beasts and dangerous snakes and insects - and that was the template for what later became 'Hell'.
Likewise, those who went to Norther lands, saw the Aurora Borealis, icebergs reflecting the sun like pearly gates and golden streets and this was the inspiration for Heaven with its blonde angels.