I don’t see a civil rights campaigner as a prophet but we have plenty of non spritual Warner’s,green peace to name just one but it still stands that most of the world never received a prophet.
I might have a different view of this as in my religion Baha'u'llah has offered this by saying..... "many a Christian divine hath objected to Muḥammad, and voiced his protest in such words: “If Thou art in truth the promised Prophet, why then art Thou not accompanied by those angels our sacred Books foretold, and which must needs descend with the promised Beauty to assist Him in His Revelation and act as warners unto His people?”
So I see it is possible that warners are other people that are inspired by the Messenger, even if they do not know the source they are connected to.
Regards Tony