Ok, maybe I missed it in the rulebooks and I could not find it in the rules questions here so I'll ask it myself: What are the rules for devout followers and how does one's followers become "devout" as it seems to be spoken of differently than dependent?

It is partly covered in the book, if it has been covered here, I cannot say. However, as I understood it originally, and understand it still it's a choice you make at the start of the game. Your followers are either devout or they aren't. You choose, and it's a very difficult choice to make that should not be made hastily...

This part I developed from the information in the book, as it is not stated specifically. So take it merely as a suggestion, not as a rule. Honestly, even the hard and fast rules of the game are not entirely hard and fast. Ultimately it's up to Fate (your GM) how it works. Basically, how I always handled the devout situation is as follows:

If your followers are not devout, they provide belief as described in the book for normal followers. If they are devout, the provide belief in the same manner as dependent followers. When devout followers lose faith, they function as normal followers. If they lose faith a second time they function as followers suffering from loss of faith

Devout followers are, by my method, automatically dependent followers and must be pampered to maintain theirdevout status. I have added an extra cushion because they are zealous to your glory and might, by allowing a "normal' belief phase rather than dumping them straight to "Loss of Faith" when they start suffering it. That probably should not be, as a Zealot that loses faith is usually a turncoat to their faith, either abandoning their god completely or swearing oaths againt them. So perhaps the cushion should be removed. I felt it was bad enough going from devout belief gain to normal belief gain. But if you choose to enforce a rub their nose in it attitude, go for Devout to Loss of faith, it'll get your player's attention.