add 's to singular nouns except Classical and Biblical names ending in s

Please tell me this is a joke! As if English wasn't hard enough - now we have to be scholars, historians and theologians in order to construct simple possessive cases.

So my colleague James gets James', and my neighbour's dog would get Brutus', but my mate Santos would have Santos's. Marcus would get Marcus' because of his connections. Chris I guess would be Chris's, but then again perhaps being a diminutive of a name having Christ as its origin would be enough for a Biblical guernsey?