We never know, they might have faith in Allah and they might think they are doing the right thing in this world of corruption.
So, a person who was lucky enough to be born in a Muslim family, and never follows any religious obligations, lives his live in ignorance and commits all the possible sins except for Shirk, murders people, steals, commits adultery, fornication everything, he will go to heaven, after suffering the punishment in Hell. So eternally, he will be in Heaven, that will be his resting place.
But a Non-Muslim Christian (Who believes in One God, Christians do), who was not fortunate enough to be born in a Muslim family, and does good deeds his entire life, never commits adultery, or any other sin, thanks God for all the bounties he has, and is grateful for every second. That person will burn in Hell for eternity. Mother Teresa will go to Hell for saving thousands of lives.
That's what you are trying to say?
Does that mean Allah has decreed who will go to heaven and who to hell, before we were even born? Since, all Muslims will go to heaven, and Non-Muslims won't. What is the purpose of our worldly existence if it's all already been decreed?
I agree that this is hard to digest but the fact that Muslims are able to acknowledge the true creator Allah and their purpose of their lives. BTW this is just a hypothetical situation.