Wha? That isn't true at all. It's done to compensate for awkwardness
I'm basing info off of some stylistic writing and grammar-related books I've read to help with my essay writing. My favorite that mentions this rule is Elements of Style, by William Strunk. I think it's rule 2 or 3 or something.
No, not true. The Jesus/Moses thing is a valid case, yes, but there are many cases where s' is acceptable for singular nouns. To say that only ancient names/etc are able to have s' is incorrect
Edit: I should say, it is nonstandard -- but not incorrect.
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