Rock N Roll has always been a very Christian based music. Jesus is just alright with me and knockin' on Heaven's door stuff. That has long been a souring aspect of it, for me, and I can't listen to many of such nonsense (almost any CCR song) without feeling uncomfy (at best).
Rock music can be a vehicle for just about any message you care to send.
True but it came out of black blues and gospel so god, or Heaven, are basic material. Kind of funny that Christians got so mad about The Devil's music, then. Eh? They just weren't listening close enough. They only heard the appeals to underage girls for sex.
John Lennon sure as hell wasn't Christian, he was an Atheist and mostly a socialist when it came to politics.
I am a fan of John but I'd have a hard time pointing to something identifying him as Atheist. "Imagine no religion" is more about religion than God. He backed down from his statement "We were bigger than Jesus Christ" as an anti-Christian statement (more of a popularity statement). His music certainly doesn't reflect a Christian belief. I'd say he was heading more into Eastern philosophy, maybe Confucian, but John really didn't get into the subject. Even politically it was more about ending war than a specific political theory.
Above all, John was an iconoclast.