"A stitch in time would have confused Einstein." - Unknown
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"From the eighth to the fifteenth centuries, the mixing in medieval Spain of the people of three religions - Judaism, Christianity and Islam - was one of Spanish society's most distinctive elements... Throughout Christian Spain there were large numbers of Jewish and Muslim people, all three groups living together but keeping their separate traditions in what might be described as an early example of multi-culturalism. This coexistence, extremely rare in European history, is often referred to by the Spanish term 'convivencia'. Here's the distinguished historian of Spain, Sir John Elliott:
"Well, as I see it, the essence of multi-culturalism is the preservation of the distinctive identity of the different religious and ethnic communities in a society. And for much of the period of Islamic rule, the policy of the rulers was to accept that diversity, even if it regarded Christians and Jews as adherents of inferior faiths. The Christian rulers when they took over did much the same, because they had no other option really, and at the same time of course, there was really no intermarriage. Intermarriage was forbidden between these communities So it is a limited multi-culturalism""
--- A History of the World in 100 Objects: Episode 62 - Hebrew astrolabe
Also, as with other commonly-mentioned romanticised historical anecdotes of religious tolerance, religions outside the ones mentioned were not treated as well.