Dude! Petra was awesome! The influence of Journey and Boston on their music was very obvious both in their sound and in their album covers. Petra paved the road in Christian "Rock" music for the bands that popular today. I had their albums "Never Say Die" and "More Power to You" and "Not of this World" and I saw them in concert a couple of times.
It's unfortunate that the Christian recording studios can't get any good producers. They might get some traction if their stuff didn't all sound so lame.
Petra's sound definately became mainstream with Schlitt (Anybody but me remember "There's Never Been Any Reason"? GREAT song.) but it had a more distinctive air with Volz.
Shalom.