Honestly who gives a shit? Whatever crazy shit he believes or not believes, he's not living to validate your own views on the afterlife or whatever. You're too hung up on a comic book writer's personal opinions.
As far I as I understand it Grant believes that we are all parts of a single eternal living organism. So in his eyes there is no 'afterlife' because none of us really have individual lives to lose and that whatever we are simply persista as part of something greater - rather than the more common atheist position that we die and end and there is nothing afterwards.
Also as I understand it Grant is an atheist in the sense that he doesn't believe in a deity. Although he does have spiritual (and maybe even religious beliefs) - in the same way that certain Hindus, Buddhists, etc are strictly speaking atheists.
What Grant believes is pretty weird. You can take it seriously if you want - but don't expect it to make sense, and certainly jumping on one individual bit of what he thinks and trying to fit it into a more conventional world view is just going to make your head hurt.