I accidentally told my churchmate why she's being ghosted by my girlfriend
Why did you tell her she was being ghosted?
My girl friend admitted to me that she doesn't like her, and I let it slip during church.
This sounds like an accident which makes it not too bad, but you should tell your girlfriend right away or else she may get upset if she has to hear it from the churchmate. While this wasn't yours to reveal, you can't change the past now and only help the future.
It don't matter, if I can't be her friend anymore but at least the message got to her.
It may have been a good thing to let her know what your girlfriend was doing to her. So at least you have that as a consolation, right? Still, you should let your girlfriend know you accidentally told her and apologize or else you may have a large fight.