I don't think thats a good idea.
For one there is the problem how to avoid people acting contrary to a gods codex doing miracles left and right. All they would need is a high enough belief skill, even if they didn't follow that god's teachings at all. To avoid that you have to codify the gods and keep track of the chars actions, which is lots of error prone work.
Second what does the skill represent? Your dedication to the god - how can you train that and how does that affect you doing miracles? How that god views you - how does the char know and how is that trainable at all? How you can sweet talk the god - that would be trainable and allow you to do miracles?
No i think it is better to keep religion out of the skills. Faction ratings work and are understandable, it is how the church of that god views you. If you absolutely have to include player characters doing miracles in a god's name, it is better done by a long string of interrelated quests that get you a special conditional glyph at the end.