Hokay, looking over the definitions above, combined with my experience of actual usage, and some knowledge of its history:
The original usage was more or less "a pronunciation used to distinguish one group from another". 2a and 2b both clearly derive from this. 1a is very like 2a, only with the extra shade of "usually regarded by others as empty of real meaning". This is appropriate since a shibboleth's 'meaning' is more contained in how it distinguishes group memberships than in its particular content. 1b derives from 1a if you add "large or prevalent" to "a party, sect, or belief".
That leaves only 1c, which essentially shades "large group of people" into "all right-thinking people". I don't actually think that 1c is an appropriate usage of the word, unless it's being used ironically, but irony often escapes poor writers.
Re: Definitions?
The original usage was more or less "a pronunciation used to distinguish one group from another". 2a and 2b both clearly derive from this. 1a is very like 2a, only with the extra shade of "usually regarded by others as empty of real meaning". This is appropriate since a shibboleth's 'meaning' is more contained in how it distinguishes group memberships than in its particular content. 1b derives from 1a if you add "large or prevalent" to "a party, sect, or belief".
That leaves only 1c, which essentially shades "large group of people" into "all right-thinking people". I don't actually think that 1c is an appropriate usage of the word, unless it's being used ironically, but irony often escapes poor writers.