If you've read the "What's Up With the Title?" section, you already know that there's some serious Christian symbolism going on in The Count of Monte Cristo. (If you haven't read that section, now might be the time.) Nowhere is that more apparent than at the book's end. For one thing, the Count literally brings someone back from the dead – at least as far as Maximilian is concerned. Also, as you might recall – especially if you've just found out what's up with the title – you know that this happens at a place called Mount Christ. Oh, and the guy who did the resurrecting has taken on the name of the place that's named after Jesus.
So, we've got this sort of literal resurrection thing, and a direct reference to Jesus Christ. "But is Valentine really the only person/thing that's being resurrected?", you ask. No way, we answer. The ending is about a broader kind of redemption and rebirth. Remember, the Count has just had a crisis of conscience (he's like, "oh man, was all that revenge stuff worth it?"). He's made some amends, but he has something special cooked up for Maximilian, a kind of positive lesson, a lesson which he puts neatly into a couple of sentences:
As for you, Morrel, this is the whole secret of my behaviour towards you: there is neither happiness nor misfortune in this world, there is merely the comparison between one state and another, nothing more. Only someone who has suffered the deepest misfortune is capable of experiencing the heights of felicity. Maximilian, you must needs have wished to die, to know how good it is to live. (117.149)
This is the ultimate lesson taken away from Monte Cristo's years of pain and suffering. It basically boils down to "No pain, no gain" or, to be needlessly wordy, "No suffering, no cherishment of life." And who should know better than the Count? It must be said, too, that it's a deeply Christian sentiment. In the Good Book, Jesus must suffer in order to redeem mankind, which is to say, only by his suffering can they come to enjoy the afterlife. If all this weren't enough, well, it helps to look at MC's parting words to Maximilian and Valentine. "So, do live and be happy, children dear to my heart," he tells them, "and never forget that, until the day when God deigns to unveil the future to mankind, all human wisdom is contained in these two words: 'wait' and 'hope'!" (117.150). He's telling them to wait patiently until Judgment Day. Monte Cristo himself already had his own sort of miniature version of it, but, as we know, it didn't go quite right. Now, just before he sails off into the sunset, he tells them to wait for the real deal.
That said, they hope, and we hope, that Monte Cristo will be back too, and long before Judgment Day. After all, everybody loves a good sequel.