A Christian may well find himself asked to justify damnation in terms which are comprehensible and make sense according to human instincts (gut feelings) about what is just.
I believe this question ought to be answerable; and without recourse either to emphasizing divine incomprehensibility (which argument can be used to justify anything at all); or to doctrines about the utter depravity and undeservingness of Man (which are completely out of harmony with the whole tenor of the Bible, and rapidly lead to insoluble paradoxes).
(Man does not, of course, 'deserve' the marvellous gift and Good News of Christ's work - but it is not legitimate to make the whole matter of salvation/ damnation into the arbitrary workings of Grace - because that is to remove Christianity from the realms of justice and desert.)
Also, this question is so basic to Christianity, that if it cannot be answered truthfully and reasonably accurately, then this is a very serious flaw.
So, how can it be just that someone be damned, with all that entails?
Ultimately, if the damned person chooses to put himself beyond the reach of salvation, then it is just that he be damned - not only just, but the possibility of damnation is a necessity for a being with free will: nobody can be forced to choose salvation.
This argument has implications: there must be a very strong, indeed total, autonomy of human choice for damnation to be just: the choice to reject salvation must be independent of circumstance, otherwise it would not be fair to damn someone because they lived in bad circumstances.
And total autonomy of choice requires that Man be considerably more god-like than Man is depicted in some Christian traditions.
(Man cannot, as I suggested above, be regarded as utterly weak, utterly sinful - else it would not be just to damn him for making the wrong choices.)
If, by contrast, Man is regarded as utterly weak and depraved in his nature (his nature being given him by God); and if all good in Men comes from God; then such a miserable creature as this depiction of Man could not justly be allowed to choose damnation - so if Man is regarded as utterly vile in himself with all Goodness from God, then such a Man is having damnation forced-upon him: which would be unjust since he is punished for that which he cannot affect.
In sum - damnation is perfectly easily explicable in terms of Man being a creature with radical autonomy of will, with the intrinsic ability to choose or to reject salvation, which entails that Man has god-like attributes such as intrinsic goodness and judgement.
That makes sense; and is I believe harmonious with the general tenor, the overall spirit or essential core, of Scripture.