The suggestion from the forum, that an dem is sometimes used instead of so doesn't ring a bell for me, but older examples confirm that it was used in a comparable fashion (the fourth paragraph and further under '1').
Today, you cannot use this construction without any context. As a sentence it will be mostly meaningless, unless a preceding or following sentence provides the necessary context. Exceptions probably contain some form of negation, like maybe "an ihm ist nichts mehr" (he's weak/done/not what he used to be).
I would interpret the forum example in the following way:
Es ist [etwas Wahres] daran, dass die Kirche schwach sein muss, um stark zu sein.
Note that an dem = daran, and that daß = dass.
Probably the following versions sound more natural:
Es ist etwas an dem, dass die Kirche schwach sein muss, um stark zu sein.
Es ist nichts an dem, dass die Kirche schwach sein muss, um stark zu sein.
Basically, the construction omits a (positive) attribute of someone/something. It's purpose is a more general setting of some statement. Here's an example from one of Goethe's works:
Wie kann man sich selbst kennen lernen? Durch Betrachten niemals, wohl aber durch Handeln. Versuche deine Pflicht zu tun, und du weißt gleich, was an dir ist.
One possibilty to extend the last sentence would be "[...] was [Gutes] an dir ist".
This - in my eyes difficult to use - phrase is used very rarely today, and I'm not sure that everyone would understand it, either. Also note that it's use is somewhat restricted to literature (see first link).
A similar construction in English would be "There's nothing to it".