ANSWER:

Allah is The Most Just and Fair.

A disbeliever who had heard of Islam and chose to reject it will never enter Paradise. 

A disbeliever who had never heard of Islam will be tested on the day of judgment. His fate will be determined according to how he does in the test.

QUESTION: Is it possible to know definitively that all non Muslims are going to hellfire? If Allah is most forgiving and most merciful, will there be non Muslims (from our world currently, not before the time of the prophet) that end up going to Jannah? Or is it true that even if someone does many good deeds throughout their life, if they don’t believe in Allah and don’t pray, they’re destined for hellfire?