This question is often asked about someone specific — a parent, a sibling, a friend who has spent a life turning away from God. The person asking is not trying to argue. They are afraid. The right answer takes that fear seriously.
And honestly: no Christian who has thought about hell teaches it with relish. The doctrine is meant to be terrible. But it is not unjust, and it is not unloving — and a careful look shows why.
God does not "send" anyone
The Western imagination pictures hell as a torture chamber to which God consigns the wicked. This is not quite the Christian teaching, and it is especially not the deeper Christian teaching.
A God who created persons capable of love made them capable of refusal. If a creature's whole self is set against communion with God, God will not coerce; he will honor the choice. C. S. Lewis put it most piercingly in The Problem of Pain: "I willingly believe that the damned are, in one sense, successful, rebels to the end; that the doors of hell are locked on the inside."
A significant strand of Eastern Christian teaching
St. Isaac the Syrian, and developed by St. Maximus the Confessor and recovered in our own day by Kallistos Ware and others, has taught that there is no separate place where God is absent. God is love (1 John 4:8), and his love fills all reality. The same divine love is experienced as paradise by those who have opened themselves to it, and as torment by those who have spent a life refusing it. As St. Isaac wrote, those who are punished in Gehenna are "scourged by the scourge of love." This is not the only Orthodox view, but it is a serious one — and it makes the doctrine even more sobering: the damned cannot escape God's love. They have made themselves incapable of receiving it.
God's justice is also his mercy
A God who simply ignored unrepented evil would not be good. But Christianity's claim is that God has done everything to make repentance available — the Cross is the measure of that love. Hell is not God's failure of mercy. It is the creature's success in refusing it.
For the one who is asking about someone they love
St. Silouan the Athonite — a 20th-century Orthodox saint — left us one of the most pastoral words ever spoken on this subject: "Keep thy mind in hell, and despair not." Hold the seriousness of judgment, and do not despair. God's mercy is wider than we can map. The same God who refuses to coerce love is the God who pursued us to the Cross. He has not stopped pursuing the people we cannot reach.
As Tim Keller wrote: "Hell is one of the costs of giving people the dignity of being something more than puppets. But Christianity gives us the only hope strong enough to face it — that the God who allows it has done everything to keep it from being our destination."