This question has been keeping me up at night.When the prophet (PBUH) preached islam in arab,he performed many miracles to convince them that this is the truth.The same is true for all other prophets.So the arabs during that time who didn't accept islam will have to answer to Allah,because they saw signs yet they were blinded.
But then when islam spread to other parts of the world during the middle ages,what is it that was supposed to convince them that this is the truth.I mean,the scientific and numerical miracles weren't discovered at that time.There were so many religions iin excistenceSo how were they supposed to know which is the truth and which isn't?What is it that Allah (SWT)is going to accuse them of?