According to an article written by Dave Sears, Mike was an atheist. (INSIDE MIKE MENTZER)
Just weeks prior to Mike Mentzer's death on June 10, 2001, Mike submitted to a Q&A session with The Sandwich. In this, his last interview, he talked about his God, heaven, and hell: (Mike Mentzer's Last Interview)
TS: Do you believe in heaven and hell?
MM: If I believed in heaven and hell, I'd have to believe in God. I don't believe in God. I have no doubt there's not a God. Therefore, there's no heaven or hell. I'm an objectivist, a student of the philosophy of Ayn Rand, whose basic tenet is that one must be fully committed to reason and reality. It's good reason that in reality it's impossible for there to be a God. There's no way, in the nature of things, for God to exist, at least as He is commonly defined.
So there's no heaven and no hell, and I have no hopes of seeing my mother and father ever again. I go through that one occasionally and it brings a tear to my eye, but that's as bad as it gets.
TS: If it was proved to you that such a place did exist and you had a ticket to go there, would you take it? Or would you stick to your convictions about the impossibility of the existence of God?MM: There is what's called a rational view of a creator. As I said, there cannot be a God as He is commonly defined. God is infinite, God is everywhere, God created the universe-that's an interesting one. There's no such thing as creating the universe or causing the universe to come into existence, as the universe is the ground of all causation! If there was a God, He would have to consist of some material substance and He'd have to live somewhere. Therefore, existence always existed, even in the context you just gave. If, as you said, it was proven somehow beyond a shadow of a doubt there was a rational creator and a life hereafter, yes, I would grab at the chance to be with my mother and father again.