The most reliable way to create a lasting community is basing it on shared religion AND costly personal sacrifices. Secularity doesn't cut it, even if demanding sacrifices.

What kind of secular communities was used in the research? The "secular community" without further specification feels a bit like a non-apple.

Maybe this is because religious communities try to solve all aspects of their member's lives, while secular communities usually have a single purpose. Single-purpose communities can fall apart when their members focus on some other aspect of their lives. For example, yesterday they wanted to save the whales or start the proletarian revolution, today they want to start a family. A religious community can satisfy a wider range of needs. Also, your relatives are often part of the same religious community.

Being religious signals trustworthiness

I imagine this is because religion has a clearly defined set of rules, and members are punished by other members if they break them. I can imagine that a christian who would steal from many people, would be unpopular within their own community. On the other hand, when a social justice warrior would steal from many people, their victims would be probably told to check their privilege, and called sexist / racist / ...phobic for trying to avoid them. Okay, I exaggerate a bit here to illustrate the point.

Being a member of a group is an evidence of a trait if the group tries to change or avoid people who lack the trait.