I'm going to tell my mother she's arrogant for praying for me
Why do you think her praying for you is arrogant?
Because I'm grown up and I can make my own decisions
I get that you probably aren't religious so it seems pointless and controlling to you but she's showing genuine concern in her own way. You shouldn't be angry about it because that's a bit rude. Just thank her for being concerned and move on.
My mom is narrow minded, she won't see that I'm trying to make my own decisions. She says I'm letting her down by leaving home even though I'm an adult now.
You're right. She should understand that you are ready to move out. If you are an adult this is really your decision and not hers.