The opinions of those whose occupation ties them to a certain set of assumptions and beliefs are not to be trusted.

This is a paraphrase of the entire extract you have posted.

Hubbard's discussion of Moses, from which this comes, does not at all point necessarily to priests. It could also mean teachers, or some one in similar professional position who might get into trouble by going up against a strongly held and powerful social opinon.

So I think the key phrase here is the author's admonition to trust only those who "have nothing to lose if they fail occasionally to gibe with the popular"

Retainer here is used not in the more common modern meaning of fee, but in the older meaning of servant -- where a servant, in this American context, is one who serves, including public officials (public servants) and other such usages.

"Committed before," means committed (to a conclusion) before beginning an investigation.

Again, in context, you go back to Hubbard's previous paragraph,

"If, however, we were asked to describe the man Moses to a jury of sane, sensible, intelligent and unprejudiced men and women, and show why he is worthy of the remembrance of mankind, we would have to eliminate the fabulous, carefully weigh the traditional, and rest our argument upon records that are fair, sensible and reasonably free from dispute,"

which is referring to his discussion of

"the fabulous, the legendary, and the probable or natural"