Does one have to know from the inside through experience the kinds of things
The ability (and perhaps inclination) to distort or propagandize is deeply human, and I see no reason to think that one is less likely to engage in such things from an "insider's" perspective than if one takes (or cannot help but be in) an "outsider's" position. Indeed, in some ways, I would expect these tendencies to be greater from "inside" than from "outside" perspectives, since those of the former group do, whereas the latter need not, have anything personal at stake. If I follow a certain religion, or have been raised to accept and engage in a certain cultural practice, or am a member of a certain ethnic group, it is natural for me to want to defend that religion, practice, or group--and to minimize or ignore the way(s) in which my religion, practice, or group may (even rightly) be seen as mistaken or wrong. Obviously, one's access to all the pertinent evidence for sound judgment may be more difficult, the further "outside" one is from the sources of such evidence, but at least one can be free of the biases inherent to one's sense of belonging to a religion, culture, or ethnic group.