Wikipedia: Apostrophe

Many respected sources have required that practically all singular nouns, including those ending with a sibilant sound, have possessive forms with an extra s after the apostrophe. […] Such sources would demand possessive singulars like these: Senator Jones’s umbrella; Mephistopheles’s cat. On the other hand, some modern writers omit the extra s in all cases, and Chicago Manual of Style allows this as an “alternative practice”.

Huh. I thought writers who dropped the ‘s’ on singular proper nouns were illiterate.

Classical, biblical, and similar names ending in a sibilant, especially if they are polysyllabic, [often] do not take an added s in the possessive […] As a particular case, Jesus’  is very commonly written instead of Jesus’s – even by people who would otherwise add ’s in, for example, James’s or Chris’s. Jesus’  is referred to as “an accepted liturgical archaism” in Hart’s Rules.

What the fuck kind of rule is that?

I’ll stick with the ‘s,’ because dropping it randomly hurts my geeky sensibilities.