Here's a pull-quote from Chesterton's The Everlasting Man. As with so many other cultural errors and historical dangers that didn't even have names then, GKC was spot-on about the ills of what Canadian pundit David Warren called "The Great Zero", multiculturalism.
... a pantheon had been set up two thousand years before by the shores of the Mediterranean; and Christians were invited to set up the image of Jesus side by side with the image of Jupiter, of Mithras, of Osiris, of Atys, or of Ammon. It was the refusal of the Christians that was the turning-point of history. If the Christians had accepted, they and the whole world would have certainly, in a grotesque but exact metaphor, gone to pot. They would all have been boiled down to one lukewarm liquid in that great pot of cosmopolitan corruption in which all the other myths and mysteries were already melting. It was an awful and an appalling escape. Nobody understands the nature of the Church, or the ringing note of the creed descending from antiquity, who does not realise that the whole world once very nearly died of broadmindedness and the brotherhood of all religions.