Mistrust of outsiders is normal...if often depressing and unfounded. Understanding outsiders is more difficult simply because of missing common points of reference. The greater level of uncertainty feeds anxiety and wariness. It presents an opportunity for contemptuous stereotypical assumptions, which has the more or less intentional side effect of boosting self-serving admiration among the insiders ("Unlike them, we break our hard-boiled eggs on the big end").
So it is in the case of mistrusting outsiders based on their religious beliefs. But I've sometimes noticed an intriguing quirk in it. Outsiders who publicly choose to not follow any religious beliefs receive greater mistrust than outsiders who follow differing, perhaps exotic, religious beliefs. According to the insiders, both types are incorrect. Why aren't both equally disturbing?
I wonder whether an important factor is an ingrained habit of measuring everyone's beliefs along exactly one dimension: the degree of conformity to the unique set of Right Ideas. This continuum underpins the continuum of mistrust. The earlier split between insiders and outsiders is a simplification of a gradual scale...or a pecking order. Everyone is sorted unambiguously into line, as if according to ticks on a ruler, or numbers marked on their foreheads, or temperature readings ranging from hot to lukewarm to cold.
The maximal insiders hold specialized authoritative religious ranks through some combination of choice, talent, ancestry, or popularity. In decreasing order, the next group is the zealots, who are fanatically faithful and eager to sacrifice almost anything. After them is the group of partisans, who are committed and dependable but not necessarily single-minded or enthusiastic. The trailing group is the nominal insiders, who are highly selective in what they believe and contribute, and who are neither dedicated nor interested to large segments of the Right Ideas.
Proceeding farther, the top group of outsiders is the dropouts, who are former insiders with faded half-hearted loyalty to the Right Ideas; they may eventually return to being insiders, although they may prefer to switch to a more fitting variant of their original beliefs. Behind them is the neutral group, who haven't ever heard the Right Ideas in an appealing form, but they're promising candidates who are receptive to discussion, guidance, and persuasion. At the minimum is the resistant group, who emphatically avoid and repel serious detailed conversations about the Right Ideas.
To be even less conformist than this, someone needs to criticize and dispute. For instance, they could definitely qualify by explicitly rejecting the essential philosophical foundations of the Right Ideas. Then they occupy an identifiable relative slot in this 1-D vision, despite their total opposition to the entire basis of it. Like the Off setting on a volume knob, nevertheless they're treated like they're actually the same as level 0. Rather than having deeply contemplated ideas of their own, they're pictured as just lacking everything that's valuable or meaningful.
Worse, regardless of their intent, their antithetical positions happen to suit the role of a villain in the insiders' vision. Merely expressing negative judgments about the Right Ideas can reduce them to targets of animosity. When worth or goodness is measured by degree of agreement with specific precepts, then someone with too little agreement—or outright disagreement—can end up "worthless" or "evil". To object too directly or forcefully is to invite unflattering associations with the other opponents that exist in their beliefs...no matter how nonsensical. ("I assume you spread subversive arguments on behalf of the infernal masters you adore?")
In effect, the insiders are under the impression that 1-D vision can supposedly accommodate principled nonbelievers more easily than believers who follow a different conception of Right Ideas. These alternative believers can't be situated using the chosen singular dimension. They don't have any clear relationship whatsoever to the (only) Right Ideas; someone who repudiates the Right Ideas does. Therefore, assuming they know their place and accept it quietly, the alternative believers tend to be tolerated at a cautious metaphorical if not literal distance.
The remedy is obvious, but I'll mention it anyway. Instead of attempting to mistakenly cram nonbelievers into a 1-D vision of religious beliefs, insiders would gain greater understanding if they broadened their perspective and permitted subtler comparisons.