[[Yes, all sin is extremely serious, and have far-ranging consequences. But are they eternal?]]
i think the point is that even in hell the sinner will still reject God- and will do so even more intensely because now the restrainer is nowhere to be seen and the person’s true character, unrestrained, will come out-
[[ If someone has rejected God in their heart of hearts, the vast universe of time will not change that decision.]]
That goes to what i said above- only now, in hell, without the Restrainer having an influence for at the very least ‘some good’ upon the unrepentent sinner, the sinner will be viciously opposed to God
These are matters beyond our understanding, but yes, if someone is utterly opposed to God, then that fundamental decision will not change throughout eternity.