but what is 'eternal separation from God?' what makes it so bad?
Because humanity's relationship with God could be its only source of purpose in the universe. Of course you're a nihilist, so you've given up on the pursuit of meaning or purpose for humanity, but it's a question that troubles me constantly.
Why does humanity need a purpose in the universe? We can fool ourselves with faith into believing whatever suits us to mask the meaninglessness of existence, and therefore be bombarded with complications and implications arising from said faith. What is so wrong with believing that our existence is unexplainable and therefore living our lives simply in the best ways we're aware, seeking no fulfillment besides solutions to that which seems prudent now and in the long term? There is happiness in simplicity and contentedness in simply living to be - for whatever flicker of time we've been granted. Ultimately a search for a "deeper purpose" will arrive to something wholly simplistic, as such.