Okay, I feel the need to clarify some things because I feel I'm being misunderstood.
1. I do not care about the achievement, I repeat, this is not about getting the achievment
2. What I do care about is that the decision, in its current form, is bad in terms of both gameplay and immersion
3. I do understand that it's very likely that the decision will be changed in the next patch anyways, but I'm going to complain about it because this is the internet
So first it's ridiculous to put the condition of "all your provinces to be muslim" as a requirement. It would be like having the confirm thassalocracy decision requiring you to have a dock built in every coastal province. It's tedious and and has no real gameplay connection. Then we get to the immersion problems. Because of the way the requirements are specifically laid out, a (for example) sunni country with exactly the 14 required provinces, all of which are shia or ibadi, sitting at a whopping 0% religious unity, could unify islam but a muslim nation that had managed to conquer the entirety of europe, asia, and africa couldn't unify islam if a single province, no matter how small or insignificant, wasn't muslim. The logic of the requirements is flawed, and should be changed.