No shit, huh.
But you do see that begs the question, not prove or disprove it?
If you're talking about begging the question of "why would they lie (about the influence of their faith on their lives)", no, I don't think it does. That of course assumes you don't have a pre-supposed answer to the question in mind, which differs from the one I offered.
The short book "More Than A Carpenter" put out by whatever institution heads up bible churches, goes about setting up this legalese argument for why Christ MUST have not just existed, but been the son of God:
1) There's far too much accurate written history from too many people, all talking about Christ, for him not to exist.
2) He claimed he was the Son of God, the messiah.
3) There are only three reasonable beliefs one can take regarding #2.
- He was lying
- He was crazy
- He was the messiah
And then it goes into all this detail of the words it was reported he said to disprove that he was lying or crazy.
The question left unanswered is this - what if the words we're left with regarding the history of Christ were simply wrong? What if they're recounting fables passed down for the few hundred years that passed since Christ's existence? It is a "reasonable doubt", and we'll never know.
The same is true in your case. They claim that faith transformed their lives. Are you assuming there are the same three options - liars, crazy people, or truth tellers? I think that begs my question, which is, "What if they're just wrong?"