When I see "closet OTD’ers" (Off The Derech) on the internet, I am reminded of my friend "Davey." Davey has a son with ODD. Not ADD—which seems to be everyone these days—but Oppositional Defiant Disorder. It isn't pretty.
Because his parents represent authority, and he despises authority, he essentially disowned them. As far as he is concerned, Davey and his wife are not his parents. He left home as a teenager and hasn't looked back in over a decade.
Does his denial change the reality of his parentage? No. He can think whatever he wants, but facts remain facts. He is just as obligated in Kibbud Av Va-eim (honoring one's parents) as he would be if he embraced them. Acknowledging one's parents is a choice, but that choice doesn't change the irrevocable biological reality. (Interestingly, Davey is cool with it; he has this "bizarre" habit of not worrying about things outside his control.)
The relationship with G-d is no different.
A person can be oppositionally defiant and claim they don't accept G-d, but that doesn't change the reality of the Creator. One can ignore Him, disgrace Him, or even curse Him—G-d is not diminished in the slightest. A person may choose not to believe, but they do so at their own peril. (One of the speakers at my father’s levayah actually mentioned that there is a mitzvah to believe in one’s parents just as there is a mitzvah to believe in G-d.) (Not coincidentally, Davey's son also rejected G-d and lives as an non-believer.)
Atheism, as I’ve noted before, is not a statement about ontology. It says nothing about objective reality because no atheist has proof that G-d does not exist. It is a statement of personal preference. They prefer not to believe. Full stop. It is a choice—a poor one, in my view—but a choice nonetheless.
The Gemara says there are three partners in a person's creation. Davey’s son thinks he can fire all three of them, but the contract was signed before he was born.
When I was a bachur, I was captivated by the books of Dr. A.J. Heschel. [I was not the only one...] While his Conservative affiliation and certain ideas he held are quite problematic from a Torah perspective, the man knew how to write like a Boss [plus he lived on the UWS like I did]. One of my favorite lines of his summarizes what I want to say perfectly:
"When man abandons G-d, G-d is not alone.
Man is."