Well, I think Augustus is pretty smart, and he does not present an atheistic worldview (or at least an inherently atheistic worldview), nor does Hazel’s pretty smart dad, whose argument about the universe wanting to be noticed perpetually is a very theistic/faith-based/spiritual kind of thing to say. (Like, embracing even the possibility of concepts like forever or consciousness that survives death is impossible in a rigidly atheistic worldview.)
Augustus’s parents, who I think are also pretty smart but perhaps not in the ostentatious way that Hazel and Augustus are, are clearly religious people.
And the last words of the book represent a moment where the author himself perhaps interjects his own let-us-not-deem-consciousness-temporary-just-quite-yet with the present tense marriage vows that could be read as a statement about celestial marriage or a marriage that survives death or etc. if you wanted to read it that way.