In the long run the answer to all those who object to the doctrine of hell is itself a question: “What are you asking God to do?” To wipe out their past sins and, at all costs, to give them a fresh start, smoothing every difficulty and offering every miraculous help? But He has done so, on Calvary.
– C.S. Lewis, The Problem of Pain
To say that those who are saved are saved through Christ is to say that all who are saved are saved by his atoning work. This work has already occurred at Calvary (and out of the tomb). Here is the math I’m using: If Christ has atoned for someone, it makes no sense to say they would go to hell (or a purgatorial conception of it) in the first place. If that were the case, Christ has not atoned for them. If he has atoned for them, it makes no sense to say they go to hell first, then get another chance to be saved through Christ.
Lewis intimates that at hell it is too late to be saved*, because the saving work has been done and already rejected; there is nothing more to do.
I believe in the Reformed view of an effectual atonement, so I come at this subject from a different angle than Lewis, but I agree with his ultimate conclusion.
* Also in The Problem of Pain: “The day is coming when you will wake to find, beyond all hope, that you have attained it [the ecstasy of heaven], or else, that it was within your reach and you have lost it forever.”