And it's interesting to note that Lennon's lyric never suggested the end of spirituality. How do we know? In a Rolling Stone interview three days before his death Lennon said this:

LENNON: “But still, the concept of imagining no countries, imagining no religion– not imagining no God, although you’re entitled to do that, too, you know? Imagine no denominations. Imagining that we revere Jesus Christ, Mohammed, Krishna, Melanippe, equally– we don’t have to worship either one that we don’t have to, but imagine there’s no Catholic/Protestant. No Jew/Christian. That we allow all… we allow it all – freedom of religion for real, I mean. For real. Just imagine it? Would it be terrible?”

Lennon held this position for the “most influential part of his life [and] can best be described as spiritual but not religious.” In what would be his last interview Lennon posited "If there is a God, we're all it." Wenner, Jann S. (2000).Lennon Remembers. London: Verso. p.48.ISBN 1-85984-376-X.

http://www.beatlesarchive.net/john-lennons-last-interview-december-8-1980.html