I definitely agree that religious outreach is a neglected but promising area of EA community-building.
I think a big part of what makes reaching out to religious groups at least somewhat promising is that a lot of them are already trying to do good. If we focus EA outreach on the general population, or most other subpopulations that EA currently focuses outreach on, you’ll likely have some people who care about doing good, and others who have different motivations. But in many religious spaces, an obligation to help others is already at the heart of what they do. And it’s a lot easier to sell EA to someone who already agrees that we have an obligation to help others as much as possible. Of course, different sects and individual religious communities have varying degrees of commitment to service and doing good, but I would imagine there’s some research already available on which groups are most oriented towards doing good (and if not, this is certainly doable research).
Also, from anecdotal experiences from friends and ex-colleagues as well as my own personal experience, I know a lot of agnostic/atheists who are involved in religious groups because they’re looking for a community, and often more specifically, they were looking for a community oriented towards thinking deeply about the world’s truths and/or doing good in the world. I think EA groups would fulfill this need for a lot of people (and perhaps relieve them from having to pretend to believe something they don’t in exchange for social support).