Forgiveness is an important concept in Christianity. Primarily God's forgiveness toward humanity.
Forgiveness is good, grand and all-around a-okay. The problem comes in the notion that we can reject forgiveness, be it the forgiveness of another person or of a fictional sky pixie.
Forgiveness is "a process (or the result of a process) that involves a change in emotion and attitude regarding an offender. Most scholars view this an intentional and voluntary process, driven by a deliberate decision to forgive. This process results in decreased motivation to retaliate or maintain estrangement from an offender despite their actions, and requires letting go of negative emotions toward the offender. Theorists differ in the extent to which they believe forgiveness also implies replacing the negative emotions with positive attitudes including compassion and benevolence."
Now, no where in that definition is the act or result of forgiveness contingent upon the offender's acceptance of said forgiveness. The only required activity is that performed by the victim.
Upon realization of this, it makes the concept of condemnation to hell (based on "rejection" of forgiveness) all the more insane.
Hell, be it a physical place or a state of mind, is certainly "estrangement" from God. Yet forgiveness requires a decreased motivation to maintain this estrangement. If God's forgiveness is infinite (or otherwise maximal) then this should result in an infinite (or maximal) decrease in motivation to maintain any estrangement; no one should go to hell, if we truly have God's forgiveness.
The two concepts are logically contradictory.
Now, people do not always practice, or grant, forgiveness in the purest sense. Admittedly it's hard to do, especially when the offender is unrepentent. It is easy to hold a grudge, to maintain estrangement, and to make your "forgiveness" contingent on damages, reparations, revenge, or an apology. Unfortunately, in doing so, you really are not being forgiving.
Then why does it seem that God, whose forgiveness should be infinite and most pure, is acting in this manner? His forgiveness should mean an end to any possible estrangement, yet the removal of that estrangement seems contingent upon additional actions or beliefs on our behalf. We need to repent, we need to believe in God, we need to adhere to some code or dogma. True forgiveness does not require these things.
God is not forgiving.