Are people sinners if they never heard God’s message?
No, not for what they could not have known. God is just and holds each person accountable only to the extent that he is truly to blame.
Sin means acting against God’s commandments. This includes rejecting His message once it has reached you. But what about those who have never heard it? And what about those who could have searched for it, but whom no one ever pointed towards it?
The best way to answer this is to put ourselves in the place of a just judge. Would we punish someone who never knew a rule and had no way of knowing it? No just person would do that. He would let him go.
And how would we treat someone who could have found out but did not? Perhaps he carries part of the blame. Even then we would hold him responsible only to the extent that he is truly accountable, and no further.
If even a human being can judge this fairly, how could God be less just? He is the source of all justice. The verses above confirm this. God punishes no one before a warning has come, and He asks no one for more than he can bear.
So there is no harsh verdict for people whom God’s message has never reached. God knows every person exactly, his opportunities, his intentions and his deeds, and He judges him accordingly.