Ringo,
Firstly, I did not assume you are not a Christian. I simply asked a question.
Im of the view that this matter of eternal punishment in hell rises or falls on (1) our understanding of the eternal attribute of God, (2) the nature of human beings, and (3) whether or not we think the human soul lives forever. If our souls are not eternal, then sins do not have eternal consequences - they are temporary. But that is not the case.
The reality is that we are beings who live forever. We are made for an eternal relationship with God, who is the eternal Being. Therefore to sin against the eternal God, reject his overtures to us, has eternal consequences.
My understanding is that when we think of sins as being temporal and not having eternal consequences, then we begin to think that eternal hell is unfair.
When I understand the eternal nature of sins, and the eternal attribute of the One against whom I sin, I understand why Jesus sacrifice for sin was the necessary sacrifice. Is it fair that the eternal Son of God had to be sacrificed for temporal sins? Thats the wrong question. The eternal Son of God was sacrificed on the cross because sin has eternal consequences.
Hebrew 7:27 states, He has no need, like those high priests, to offer sacrifices daily, first for his own sins and then for those of the people, since he did this once for all when he offered up himself (ESV).
So the reason why eternal punishment in hell is fair is because: (1) Of the nature of God, (2) The nature of human beings, and (3) The eternal consequences for human sin against the eternal God.
God is absolutely just and always does what is fair and righteous. Thats why the consequences for sin for the unregenerate are eternal punishment in hell.
See:
"How is eternity in hell a fair punishment for sin?"
Oz