It would seem more reasonable to use them being an athiest /agnostic.
God damn it, is blasphemous, so is being athiest to religious people, so swear away. Gawd damn it.

i tend to substitute the F word, probably too much. but there are still some shitty, dumb things that happen for which godamnittohell works well.

I don't believe in gods, the soul, heaven or hell, spirits, karma.. pretty much all that bullshit, but I'll still use sayings related to them. Like hell I am going to let the religious claim words as their own. To me, by allowing them sole ownership is almost like passively admitting to their belief.

I use these phrases ALL. THE. TIME. They are the words I used to express anger and frustration, so I continue to use them in that context. I feel a bit hypocritical using them, since there is no god, but also rationalize that since there is no god, there is no cosmic supervisor to get pissed off by my using them. I figure it's a draw.

It maybe an unpopular opinion, but I find the whole concept of hell fire and damnation so utterly horrific that even as an atheist who has no belief in any such idiocy, I find wishing that degree of even imaginary harm and suffering on any utterly deplorable.

I grew up in fear of hellfire and Satan, and as an imaginative child/infant I had many a nights sleep ruined thanks to my mothers filth ridden salvation and damnation lectures, bible readings and horror stories.
It damaged my mental and physical health and left me with a life time condition that I still struggle to control today and nightmares I can only escape by channeling them in to fiction.
So I don't personally use those those expression because of the history of hate and loathing behind their creation.
Christians disgust me when they find anyone worthy of that punishment for any reason and then worship the god who inflicts it.

Not to be flippant or disrespectful, but who cares really? They are part of the swearing vernacular and theists use them all time even though they are supposed to.

Sure, they are just words, part of the culture. Swear away... I do.

I'm not afraid to continue using them...Goddammit!