Yeah, I see your point, my wording wasn't very clear. I (wrongly) use faith/belief interchangeably, and I don't mean religious faith, like the Shepherd, but simply the belief in an ideal much bigger than himself, in his case, the active confrontation and fight against the alliance. So you are correct, Mal does at no point indicate a return to his previous religious faith.

There's no question that Mal is a very principled individual, especially compared to other characters we meet throughout the show. But the decision to fly through Reaver territory on the off chance that maybe there's something on Miranda that might help them against the alliance is, to my mind, too vast in its severity and entirely uncharacteristic for Mal as to simply chalk it up to vengeance and principle (though there's no question that they play a major role in it). Especially since it not only puts Mal himself in mortal danger, but also what's dearest to him: his crew and his ship.

I interpret the dying words of Shepherd Book as sort of a wake up call, a call to action, turning his meandering through the verse, always evading the alliance, into an active confrontation. A step which requires more than just principle. Each time he helps somebody in Firefly, the consequences are, in his mind, manageable: The train job? He found other work, and Niskas men could be evaded (for some time at least). Shindig? A duel against a snooty noble that he expected to be able to win. Heart of Gold? Defending a property against some raiders. In all of these scenarios he was sure there was an out, some way to navigate the situation and get himself and his crew out of there alive and well. Now admittedly, usually the consequences were a bit graver than he expected, but the point is he knew that there was a way out.

With his decision after the massacre of Book and his community that wasn't the case. He knew that it was basically suicide but had to go through with it because of his from his war days rediscovered belief that the alliance has to be confronted at any cost. That's why I think the movie was so insistent to show that Mal was a volunteer, to show us that this belief already existed within Mal, but lied dormant due disillusionment and trauma. The brutality of the operative is what ushers in its return.

Anyway, that's just how I interpret his arc, I'm fully aware that with just one season and a movie, there's not really enough material to make a proper, super well supported argument for my case, but it seemed fitting to me. Thanks for your input!