I don’t view the end as entry into a place so much as becoming your true self.
The atheist, by his very presumption assumes his not-self (the other that he faces) is mindless, intentionless, purposeless matter. It is in its every essence nothing like personhood or self. So in all the exercise of his quest to become himself, he views this endeavor as essentially one of being alone in the cosmos without hope, purpose or destiny beyond what he creates for himself. His endeavor is essentially sterile because it is self-oriented.
On the other hand, the theist views self as relational with Another. The nature of self is then one of being in relationship with Living, Eternal Being, Personhood.
I suspect being an atheist is not so much a choice but a symptom of one’s current state – one of being a haploid cell (an infertile existent) rather than a zygote. Making oneself ready to receive Other is required for fertility to occur. My advice would be to delve deeply into what it means to be essentially sterile and experience the existential angst full on – it may give a glimpse into the void that needs to be filled and provide the impetus required to make oneself at least somewhat open to receive.
So to answer “What likely harm has the atheist done to himself?” the clearest indicator might be to show the difference between an infertile egg and a fertile one. The difference is that one becomes something new and unique (a chicken) because of (name removed by moderator)ut permitted from Other, while the infertile egg remains simply what it is (an infertile egg.) Unfortunately, we can see what happens to infertile eggs left on their own beyond their shelf life