Some markets are hard to serve if you are not in the market segment yourself. I'm not sure that an atheist developer "serving the religious market" with a Bible app is necessarily going to be successful. It helps to read the Bible on a regular basis yourself (on your device and off) so that you know what people want. For example, it is often slower for me to find a specific passage in the iPhone Bibles than in the physical book, which is problematic if I am trying to follow along the sermon, since the passage is often finished by the time I locate it in the app. This might not be something that an atheist developer would discover.

Likewise, if I tried to develop a Qur'an app, it might not work so well, because I have no idea how a Muslim uses the Qur'an. Of course, I could go talk to a bunch and write an ok version, but I'm sure it wouldn't be as good as an app that a Muslim would write (given a developer of equivalent skills).