
The idea is that precisely because of Christendom and the Enlightenment, the culture of individual European nations is much more similar to that of others than, say, two countries in Sub-Saharan Africa or any given region of Asia. And Europe's entire population is (and has always been) lower than that of the individual nations of China and India, both of which have multiple major religious traditions.
My point is that the actual level that culture plays out in lived experience is at those regional distinctions, not at some arbitrary construct of white culture. To say an Irishman, Scotsman, Englishman, and Welshman even have the same culture would be to find yourself immediately the enemy of all 4
Probably the single best answer to that question is "Western art music", or what is referred to as "classical" but including the Baroque and Romantic eras. Until perhaps a century ago, composers from Russia to Portugal were composing the same general type of music for the same instruments.
If you want to go back before that, Gregorian chant has existed across Europe for 1500ish years. I don't think it's entirely unfair to consider mostly the aristocracy's art or music, because they were the ones travelling all over Europe or at least being exposed to ideas from all over Europe. If you consider folk music and whatnot, practically everywhere on the planet would turn into a distinguishable subset, and "Japanese" or "Mexican" or "[country]" culture would be no realer than "European"