John the Savage is our view into the dystopian world in which the story is set. He is someone recognizable with whom we can identify. John's whole moral outlook sets him apart from the rest of society while at the same time giving us someone to root for. Furthermore, John acts as a kind of prophet, drawing upon the wisdom of the past to provide a basis to challenge the soulless, technocratic society of the World State.
As a so-called "noble savage," John has led a more natural existence, closer not only to his immediate environment, but also to what makes him human. This means that he is largely immune to the moral corruption with which this nightmarish world is contaminated. At the same time, John, being human, is subject to temptation, which he must constantly fight if he's to retain his purity. This merely adds to his status as a Christ-like figure.
Although life in the World State is, on the whole, rather repellant, it does have its attractions, and many readers of Brave New World will doubtless have been tempted by certain aspects of life in this fantasy world and wondered what they would do if they were in John's shoes. Once again, this establishes a firm connection between John and the reader, pulling us deeper into the story as we contemplate the profound moral dilemmas raised by our hero's predicament.