If it can truly be tested, then it should be tested! But how do you truly test it? Even if God isn't real, acts like prayer give you emotional support and that can have a very real effect! So if you merely want to test whether an act like prayer is effective, then that's simple and probably already done and also probably shows some effect. However if you want to prove that prayer in a particular method to a particular God is effective, that's tricky. If someone can design an elegant enough study then it should be tested!

If they could build a mathematical model and simulations...Why not? Seems like a low-effort experiment.

You’d need to run a double blinded randomizes controlled trial with enough patients to test the effect.

And it would be wildly unethical to use prayer which by any reasonable understanding of cancer biology would not work.

Why does it have to replace therapy? One arm can be prayer + therapy and another can just be therapy

Ok, that would address the unethical part, but still, does that sound like a good use of limit research dollars?

This is one of those instances where judgement is a good thing. Don’t run such a silly trial. Spend the money on something that actually has a chance to work.