Why would monarchs care a lot about religion?

My best quess/answer would have to eb 

monarchs would have cared about religion because a unified kingdom(or what ever they rule over) is stronger and more feared if they are one in as many aspects as possible. This is because what if (what ever they rule )inst in perfect unity then they have a weak spot the enemy could find and make the whole (whatever they rule)come crumbling down quite fast and easily.

hope this helps and have a great day ^_^