Indeed, which is why I think the question of this thread does not make much sense (sorry, OP). How do we even define "fame"? Jesus came up for example and I'm trying not to get into religious debates in this thread, but regardless of whether he existed or not in reality, is the person whom people have in their minds as Jesus today the same as the possible real figure (or several figures) behind this story? Is really the real Jesus that is famous or is it his myth? Of course, the answer may depend on what you personally believe about him.
And if we talk about such figures as Jesus. If he existed during his era he was of course not as famous as any living celeb is now. Today there is global mass media, Internet etc., while the real Jesus probably was only famous in and around his own town or so. If he was famous at all outside of his immediate followers. The fact is there are no contemporary writings about him and contemporary Romans apparently did not consider him significant enough to have even a note about him. So he did not seem that famous to me. The gospels were all written several decades after Jesus supposedly lived so how credible can they be considered? It's like we had to reconstruct Michael's life based on tabloids.
Due to technology, video, recordings etc. - as long as those exist - I think people will know more about the real Michael than the real Jesus. Even if unfortunately Michael's story too is polluted with myths and untruths "thanks" to tabloids, false accusers etc. - but the factual records will always be there as well. Unlike about Jesus about whom we do not really have any factual records. So can you then really call Jesus "famous"? It's the Bible's version of Jesus that is famous due to the fact that because of historical circumstances Christianity went on to become a major world religion.
On the other hand, the Biblical Jesus has been famous for 2000 years, so I think if we add all the Christians (and later Muslims and others) who lived in the past 2000 years, I think we would get a number of people that is much bigger than the people who live today and so despite of global mass media today - due to 2000 years of head start - no current celebrity can compete with the fame of the Bible's Jesus.