I have an article coming out in a week or so that tackles that very question. I apply this existentialism to Matt Dillahunty’s atheism, producing what I think is the best response to the new atheist’s challenge for the theist to produce strong evidence in support of her religious beliefs.
I’m aware that theists often resort to scaring secular folks back into religion, by reminding them of the horror of the non-naive atheistic worldview. Actually, the theist has little advantage there, since the non-naïve, mystical version of monotheism is just as unsettling to the average, exoteric monotheist. For example, the naïve theist gets to read the Bible as literally true or as inerrant. However, the more cosmopolitan and philosophical Christian understands how myths and metaphors work and is aware of the historical-critical approach to the Bible.
We should beware of strawmanning either non-naïve atheism or theism. The two virtually meet in the middle, which is to say that the mystic’s God is equivalent to nonbeing (to pure no-thingness, God being other than any finite thing, by definition). I’ve written about this elsewhere (link below).
I argue for something like the objectivity of aesthetic values, given a pantheistic conception of nature’s creativity (links below). That’s how I’d reconstruct morality after the death of God.
You jump from the subjectivity of values, though, to the absence of truth, given atheism. I’d argue for a pragmatic account of truth. We know what’s true because it works.
Again, the theist has no advantage over non-naïve atheism here, since if moral values are grounded in the person of God, they’re likewise subjective. “Objective value” is usually oxymoronic. Aesthetic values come off as roughly objective, though, because they’re arrived at by a similar act of mental detachment than the one we perform in being objective.
Still, in the upcoming article I offer something close to your third paragraph as the best response to new atheism, namely a pragmatic defense of theism. This would entail that theism is useful (and thus perhaps true in some pragmatic or subjective, Kierkegaardian sense). Theism would be a noble lie, a myth we need to hold society together. But not everyone is naïve. Some people are philosophically awakened and duly marginalized because they see through such myths, especially the outdated ones that require too much childlike naivety in suspending our disbelief.
In any case, I’m with Nietzsche in searching for a new hypermodern faith, for one that would be superior to both neoliberal secular humanism and to the anachronistic religions such as Christianity and Islam. The Eastern religions are already superior at least to the latter, but they’re problematic in other ways (a link below, if you’re interested).