I’d agree there’s an interrelationship between theism and atheism. The most responsible, tenable form of theism is functionally equivalent to the atheism. That’s the mystical or liberal kind that takes the personification of the First Cause to be a vain metaphor, not to be taken literally. And that leaves room for the synthesis you spoke of. By contrast, the non-elite, exoteric, mass or vulgar form of theism is literalistic, dogmatic, and sanctimonious about some interpretation of a myth or scripture.

But atheism stands apart from theism in the former’s positive aspect—because there’s no such thing as just atheism. Atheism is the answer to a relatively narrow question. Atheists have a nontheistic worldview such as secular humanism, philosophical naturalism, neoliberalism, consumerism, materialism, socialism, transhumanism, and so on. Those worldviews diverge from the theist’s worldview, from Christianity or from the other religions.

In particular, while the secular worldviews may have their flaws, they’re superior to theistic religions in two respects: they’re not hopelessly outdated and they’re not anthropocentric. To be sure secular humanism, for example, may be hubristic, but it doesn’t entail so childishly that the universe is made for us or that our species figures in a cosmic plan.