Many Christians, not just traditionalists but liberals too, have testified to the power of the Gospel to transform individuals' and families' lives. It's a wonderful thing when such a transformation happens.
But traditionalists often claim that the fact of such transformations establishes the truth of their various add-on doctrines — the Incarnation, the Atonement, and so on.
There are at least two things wrong with their thesis.
First: All sorts of different religious beliefs and practices have positively transformed lives. We don't understand how it happens. We can say that such transformations are by no means unique to the Christian Gospel, let alone the traditionalist version of it.
Second: The Gospel transforms the lives of only some of those who hear it. We don’t understand why that is, either. It’s reminiscent of how doctors used to treat common peptic ulcers with diet changes. Sometimes that worked, but no one knew why. Now we do know that common peptic ulcers are caused by bacteria, and sometimes diet changes can alter the stomach environment enough to inhibit bacterial growth. (Of course, we also know now that it's usually more effective to just administer an inexpensive antibiotic.)
These two things make it hard to accept the traditionalists' claim: that the Gospel's transformation of some lives is evidence of the truth of their particular spin on it.