I think its just a human characteristic to want to know whether someone you have just met can be trusted or if this is an untrustworthy or even a dangerous person. This "need to know" is left over from "kinship" ties or tribal affiliations, from early human development.

One of the things that at least appears to make an unrelated stranger potentially trustworthy is if the stranger shares the same religious beliefs as yourself, or at least, if the stranger even professes to have well-known, familiar religious beliefs.

The idea of a "religious person" conveys at least the appearance that the stranger adheres to common notions of "good" vs "bad" and "right" vs "wrong" behaviors. Shared religious beliefs at least give the appearance of a distant "kinship" or trustworthiness.

But the kicker is that a truly nasty, psychopathic individual who intends harm can give the APPEARANCE of sharing your religious belief for the purpose of lulling you into a false sense of trust.

We human beings like shortcuts: we like the easy way; that's why we're gullible and easy to fool. It takes time and effort to actually get to know someone, and discover by a person's words and deeds over time whether they're genuinely trustworthy or not, and it takes no time at all to just "buy" a person's declaration that, "Hey, we're "brothers in the faith" and so you should trust me."