This has been discussed before on this Board. It’s a matter of style. Biblical references do not use the double “s.” For example, Moses’ laws, Jesus’ beliefs, etc.
I always use the ‘s with these names. Where does it say not to? It would sound bizarre, or substandard, to leave the ‘s out of a possessive. Like "Here is my friend’ phone number." Or "‘Spare your country’ flag,’ she said"
I actually had to look this up for work this past week, and was surprised to find (according to Garner’s Dictionary of Modern American Usage) that “Jones’s” and the like is the preferred form. I, too, had been taught not to use the final “s.”
Garner does note that stylebooks for newspapers (which tend to be more concerned about space) do allow writers to drop the “s,” but “most authorities who aren’t journalists demand the final -s.”
Current copyeditor checking in and quoting from the Associated Press Stylebook:
"Plural nouns ending in s: Add only an apostophe …
"Singular common nouns ending in s: Add ‘s unless the next word begins with s: the hostess’s invitation, the hostess’ seat…
"Singular proper names ending in s: Use only an apostrophe: Achilles’ heel, Agnes’ book…
“Special expressions: The following exceptions to the general rule for words not ending in s apply to words that end in an s sound and are followed by a word that begins with s: for apperance’ sake, for conscience’ sake, for goodness’ sake. Use otherwise: the apperance’s cost, my conscience’s voice.”
That’s a pretty good, intro-level source, xash. I would say that the only thing they got wrong was when they claim that it is the only noun which does not form its possessive with an apostrophe. This is untrue; it is not a noun.
Yes, and of course, all the pronouns act in the same fashion: he-his, she-hers, me-mine, you-yours. The spelling is different, but these are all possessive with an apostrophe.
Whether you add an “s” after the apostrophe in nouns ending with “s” depends upon what style manual you subscribe to. Personally, I like JonScribe’s AP, but I bet the Chicago Manual has different rules. Strunk, who has been acknowledged as the leader in the English language, has the rule I previously described.
Chicago Manual of Style
6.12 The possessive case of singular nouns is formed by the addition of the 's.
6.15 Proper Names. The general rule covers proper names as well as common, including most names of any length ending in sibilants.
6.19 Traditional exceptions to the general rule for forming possessives are the named Jesus and Moses (their emphasis)
Washington Post Desk Book on Style
“Use 's to form the possessive of singular nouns, proper names, and nicknames ending with a sounded s” (i.e., same rule as Chicago Manual)
So we have both the book publishing and the newspaper world covered. The logic is simple: 's for singular, ’ for plural.