How to argue this point?

[Fidel_Castronaut]

Simply put; Evidence or GTFO.

Listen, people can claim all sorts of things, and believe them.

If he wants you to believe it, he'd need to provide verifiable evidence of the initial ailment, the method used to treat it, failures of other (medical) treatments, and what he defines as 'treated'. After all, a 'bad back' is subjective. What does that mean? At the very least he could get you the diagnosis of a bad back and some sort of confirmation from a clinician that his back problems are a thing of the past then we could at least infer that something has happened.

Would also help if he could get this pastor to do this again under a controlled setting where the experiment and the results can be analysed and bias discounted.

If he can do all this and consistently produce the same results, then, and only then (I presume) will you believe there is an element of truth to it.

Also, welcome!