Reworded: "I'm so blown away by what I'm experiencing that it's hard for me to believe it's even humanly possible, what you're doing."

I can't imagine anyone taking this as an insult. If that person exists, I hope I never end up in the unfortunate situation of paying them an incredible compliment.

Reworded: "I'm so blown away by what I'm experiencing that I think there's a very real possibility that you don't deserve full credit for this achievement."

If you're ever in the position to give someone a hyper-superlative complement, you could simply use one of the many hundreds of constructions that don't involve doubting the skill of the recipient.

Addendum to your rewording "... BECAUSE I'M SO MOVED THAT BEFORE TODAY I'D HAVE THOUGHT IT WOULD HAVE TAKEN A FUCKING DEITY TO DO WHAT YOU JUST DID."

Edit:

Further, I don't believe in God but I also don't believe I've ever felt I deserved "full credit" for an achievement in the way you're describing.

Calm down mate, I'm merely disagreeing with you. I do understand your point, and I do think that your interpretation makes for a reasonable hypothesis, but it's my opinion that it's less plausible given the exact wording originally provided.

That's not at all what he was implying.

As a musician, I would view that as the ultimate compliment.

I wasn't saying it wouldn't be received as a complement, after all, it's just as likely that it was offered in all sincerity. I'm just saying that the literal implication could be construed as an insult.

I would not think so. My impression is many artists (musicians too) feel inspired by a higher spirit, so to speak. Personally, I am not a believer, but I understand how it can feel when one is.