Lifeisgenerallyfun
Islamic perspective here - Gods teachings have been passed on since Adam. Prophets were sent along the way (some of which came with scriptures) to guide man to worship one God.
Thus people were to ‘follow’ the most ‘recent’ prophet of their ‘time’.
We believe that the same message has come from the beginning, except that the passage of time has not 100% preserved the text, so you have Judaism, Christianity etc.
All prophets called to people to worship one God, but the final prophet was Muhammad (peace be upon him) after which there are no more prophets, but the Day of Judgement.
Undoubtedly there will be some people who the message may not have reached, God will be the judge of that. But for anyone who reflects, the sign of God is in His creation - is, the world around us etc; so even if they are unaware of the minutiae, people will recognise a creator or some higher power.
Of course, these things constitute the finer details - they are not entirely faith based - for us, we accept the Quran as the word of God as truth - if an individual accepts this they will in turn accept its contents as truth - heaven, hell and so forth.