As another owner of the achievement i going to say that i got it as an intermediate objective back in 1.17 on my way to the Orthodox One-Faith.
I dont know why one would quit afterwards.
The modifier it gives is amazingly strong and if you keep your Missionaries busy its not hard to get.
Especially if we assume Ottomans, with which getting it around 1520 isnt difficult
I'm talking about the achievement mentioned by @Dominion - 52 Garhs - not Dar-al Islam.