"Do not be afraid of those who kill the body but cannot kill the soul. Rather, be afraid of the One who can destroy both soul and body in hell." Matthew 10:28

Men can destroy us, and our bodies. But only God the creator can destroy our souls, so He is the one we would be wise to fear. When he casts us into the lake at the end of time, I believe that it's more than a prison. From what I've read, it seems to be more like a garbage disposal, destroying that which is thrown in. I could be wrong though.

I think u/Unreasonable is basing the whole non-eternal thing on this verse, or another verse like it, perhaps. The void, hell, whatever you want to call it, is eternal, yes. However, I think there's more to it. It's a sort of annihilation, a permanent destruction, that could be viewed as non-eternal much like a piece of paper will not last the universe's life; The constituent atoms have been there since the beginning, and they will be there in the end, too. However, the piece of paper itself is not eternal.

I'm not an expert, this is just my interpretation. God bless!