Sphere is important, but a guy who's sphere Rage might just be an angry guy, a guy whose anger colors every decision he makes. He's not necessarily religious.
A sphere rage guy who is religious probably follows Ysal, and from what I get, will focus his decisions not just on being angry, but by considering how his anger allows him to shape the world.
By the same token, a lot of religions kinda focus on how your sphere relates to you in the world instead of just in your own personal dealings and headspace.
Being a good follower and getting tattooed will generally either mean toeing the line of the religion in a way in which it is basically understood, or bringing a fresh approach that maybe even the IMM didn't see coming.
If you can do that, you should also be able to get empowered.
I feel like most of them are looking for excuses to reward followers, and I've never been given a task that I thought was impossible when dealing with any of the IMMs.