How to get rid of mildew on walls?

Since I had to be hospitalized for mold poisoning and people die from black mold exposure, especially the young, old and immunodeficient people:

This is a job for pros because dead mold spores still are toxic and they’re working their way into the rest of the building because the spores fly everywhere through the heating & air conditioning.

First, find the source of the water and stop it.

Actually before that, make sure this room is pulling air from the clean part of the building and exhausting it outside.

Mold must be killed and removed, not encapsulated. That means do not paint over dead mold.

Be careful and be safe.

Peroxide kills it but then paint with a kilz based paint

White vinegar is better than bleach for this type of job. First place Damp Rid in the area or get a dehumidifier (depends on how serious the problem is and if it is a large space). Until you resolve the damp issue that is causing the problem, you are wasting your time. Once that is handled, white cleaning vinegar either full strength or 50% diluted, again depending on the severity of the job. Then prime with Kilz Original.

Call in the pros. Bleach doesn't kill the mold spores, it's an old wives tale. Vinegar will work.

https://www.servicemaster-restoration-cleaning.com/2017/03/mold-myths/

Yours sounds like an infestation that has been neglected over time. While the above method works for surface mold, I guarantee there is more going on than what meets the eye. No sense on risking the health of the church members. Call in the pros.

I agree with Cheryl, don't take chances with anyone's health in the church. Have the moisture problem found and fixed and all the moldy areas removed and replaced before even thinking about painting.