The Chicago Manual of Style has changed some of the rules for forming the possessive of proper names.

CMOS 15 said that names ending in an “eez” sound (like Xerxes) got only an apostrophe, without an extra s after it. CMOS 16 no longer makes that exception. The recommended punctuation is now Xerxes’s.

Note that CMOS 15 and 16 both suggest that the possessive form of Jesus should have an apostrophe-s: Jesus’s.

The Christian Manual of Style, however, says that when there are two s sounds together at the end of a name (e.g., Xerxes, Moses, or Jesus), only an apostophe should be added to form the possessive.