Won't see a dr. Gets defensive, and says no locks or alarms can keep them out. Troublel seeing now (even more) Trouble driving. Accuses church members too of stealing. She refuses to see a doctor. Gets VERY defensive if you try and reason with her. (79yo)
As the others have mentioned, you need to get her to a doctor ASAP - do whatever it takes to get her there. Your mom needs the help of a doctor to be able to deal with this. It's only going to get worse.
I've seen suggestions here where you tell your elderly parent that they have to see the Dr. for insurance purposes, that it's just a checkup. You might try that.
It's futile to try to reason with someone with dementia because they have lost the ability to reason. It might only agitate your mom more if you try to tell her that what she believes, that people are stealing from her, is not true. In her world, in her mind, it's very true. That's her reality. It's probably very scary for her.
Instead of trying to reason with her try comforting her. Let her know that you're not going to let anyone hurt her or steal her things, that you keep the house locked up tighter than a drum. Reach into your bag of therapeutic fibs and tell her that the police are doing extra patrols on your street and that you won't let anyone touch her things. Reassure her.
And if you can get her to a Dr. see if he will prescribe an anti-anxiety medication.