I believe it's called Zardoz, with Sean Connery.
I need to rewatch Zardoz to see how horrible it is. I saw it as a kid and just thought it was really weird.
Speaking as a connoisseur of Cold War era post-apocalyptic dystopianism, Zardoz is great fun.
That's why it was paid homage by Rick & Morty.
Nearly pissed myself laughing the first time I saw that!
I remember that giant floating head thing, and some plot about immortality. And Connery hiding inside the statue's mouth. I was really young when I saw it, and at the time didn't think it was bad, just strange.