This was also prompted by my reflections of the ongoing Rabbi Slifkin controversy.
And also by a friend who once made the following fantastic comment about why a shidduch didn't work out:
"She couldn't even understand the depth of my kefirah, how could she possibly understand the depth of my emunah"
(this also assumes that spouses understanding each other's inner religious worlds is a value. it also assumes that most people have inner religious worlds. if you don't, stop reading here)
Let's say that am ha-aretz A, let's call him Berel, believes that HKB"H is an old man with a white beard who sits in the sky, kind of like Zeus. And he's very frum, davvening to Gandalf or whatever thrice daily, etc.
I'm not one to knock Berel. He's a good guy. But in a moment of honesty, I don't believe in his God. In my own view of things, his God doesn't exist. There's no old man in the sky.
Oddly enough, that makes me a kofer in Berel's eyes. And he's right. I deny what he believes in. And the truth is, if I thought that Berel could handle a more sublime, mature (there's that word again) type of belief, I might try to lead him toward greater emunah - a theosophy in which HKB"H's relationship with the world is closer, more sublime, more immanent, more inward. But if it backfires, then I'm forever a heretic in Berel's small mind.
Let's translate this into historical terms. Human knowledge has gone through some changes. We have been forced to confront questions that has refined our view of the universe over the millenia. So when somebody like, let's say, Maimonides, thanks to an Aristotelean or Neo-Platonic worldview insists upon God's non-corporeality, he has taken emunah to the next level. Before Rambam, there was an awareness that God was 'invisible' or 'unseeable', but 'incorporeality' wasn't in the otzar ha-milim. Thus, Rambam was a heretic at one level - he denied the existence of the object of worship of many, many Jews - but paved th way for a different kind of faith - one that has become the norm.
Thus, R' Kook can write about the holiness that pervades 19th century atheism. He saw the resistance of the human psyche to an enslaving or dominating commanding Presence, a phenomenon which sparked the atheisms of Freud and Nietzsche and some characters in Dostoevsky, as a gateway to emunah which saw human will itself as a reflection of Divine will. Thus, he denied the tension between G-d's will and man's will. Nietzsche's Emancipation Proclamation from God's enslaving spirit opened the door for God's ennobling and sublimating spirit. V"acamol.
It's a pattern - kefirah opening doors for greater and greater emunah.
Back to Berel. I drop a pen and ask Berel, "What made the pen fall?". He says, "retzon Hashem". I say, "Gravity". In his view, God's Presence is more palpable superficially, but external to the Universe. In my answer, God has receded into the background, but thereby becomes the ground and matrix from which all of reality rises. Who has greater emunah? I know what R' Kook would answer.
One final point: none of this discussion pertains to what God 'is', only to how we percieve Him. Emunah is a human scale. In the developmental model that I keep going back to, one view can be more mature than another - it reflects a later stage of cognitive or emotional or moral development. One's aspaklaria can be clearer than the other's. but both are views of Truth, not Truth itself.