First, the organization must have an original steering body made up of "true believers" in a shared mission and vision. This body must make preventing the short or long term erosion or dilution of its mission a priority. Meaning that there's awareness that this particular corruption is a possibility from the start. Also, an understanding of the typical mechanisms through which it can happen (usually very human).

Second, as part of its survival strategy the steering body has the responsibility to identify other genuine true believers and integrate them in its makeup. There must be built-in assumptions that many outsiders will be attracted to its powers of influence and will try to infiltrate it.

Joining the steering council should thus be done primarily based on "culture fit". Admittedly a rather segregating practice, but one which in this case comes with the advantage that certain signals are just hard to fake on the long run. So, although many could try to dress, look, talk, or walk the part for a while, there will always be some shibboleth that trips up impostors.

I foresaw some problems to this structure that I haven't yet worked out, but it feels like a step in the right direction, if I had to come up with a solution.