It would be almost impossible for us to list every single Biblical allusion in this book. As a devoted theologian, C.S. Lewis was well-versed in Christian symbolism, and even his simplest books are densely packed with religious references. So let's focus on two categories of Christian symbols that provide structure for the plot of The Voyage of the Dawn Treader: metaphorical versions of the sacraments and allusions to God or Christ.
OK, we realize that, depending on whom you ask, only the first one of these three things is technically considered a "sacrament." But all three have been sacred processes in the life of believers throughout the history of the Christian church. Baptism you might already know about: whether it involves sprinkling, splashing, or dunking, it's about using water to symbolically purify the individual believer. In The Voyage of the Dawn Treader, Eustace goes through something that looks a lot like baptism when he meets the great Lion, Aslan. Eustace tells Edmund:
Then he caught hold of me – I didn't like that much for I was very tender underneath now that I'd no skin on – and threw me into the water. It smarted like anything but only for a moment. After that it became perfectly delicious and as soon as I started swimming and splashing I found that all the pain had gone from my arm. (7.44)
Aslan's immersion of Eustace in the pool is an obvious-if-you're-looking-for-it, hit-you-over-the-head reference to the kind of baptisms that John the Baptist and Jesus of Nazareth perform in the New Testament.
Eustace also goes through a process of self-confrontation and rebirth that reminds us of contemporary Christian ideas about being "born again." His transformation into a dragon seems to simply show on the outside what has already happened to him spiritually: he's hideous, buried in layers of rough nastiness. Before Eustace can be tossed into the pool (i.e. baptized), he must peel off his outer shell of meanness and selfishness and get back to his true self. One of the particularly Christian-sounding points that the book makes about this process is that Eustace can't do it alone. Whenever Eustace tries to peel off his outer shell himself, he only gets a thin layer, leaving lots more behind. Aslan has to do it for him in the end, and it's far from a painless process. This is not unlike the Christian idea that Jesus steps in and transforms the believer, because the believer can't avoid sin or become pure on his own.
And Eustace isn't the only character in The Voyage of the Dawn Treader undergoing a spiritual process. The ship's voyage to the extreme eastern end of the word in search of Aslan's country is much like a pilgrimage to a sacred site – or like the metaphorical "pilgrimage" that every Christian believer goes through while trying to reach heaven. Although everyone on the ship takes part in this pilgrimage, it's Reepicheep who devotes himself to it most fully and who gets to travel the last leg of the journey alone. Our last sight of Reepicheep is of him disappearing at the top of a strange wave:
The coracle went more and more quickly, and beautifully it rushed up the wave's side. For one split second they saw its shape and Reepicheep's on the very top. Then it vanished, and since that moment no one can truly claim to have seen Reepicheep the Mouse. But my belief is that he came safe to Aslan's country and is alive there to this day. (16.56)
Reepicheep, who sails straight into Aslan's country, is reminiscent of Elijah, the great Christian prophet who didn't die but instead was "translated" in bodily form into heaven.
If you've read The Lion, the Witch, and the Wardrobe, you've already gotten the whole Aslan-is-like-Jesus thing. But if the first Narnia book didn't hammer it home enough, C.S. Lewis definitely spells it out for us at the end of The Voyage of the Dawn Treader. Aslan tells Edmund and Lucy that he exists in their world, too:
"But there I have another name. You must learn to know me by that name. This was the very reason why you were brought to Narnia, that by knowing me here for a little, you may know me better there." (16.76)
In effect, Aslan is revealing to Edmund and Lucy that all their adventures have been part of his grand plan for their lives, educating them spiritually and preparing them for development as good Christians. Yes, the other name that Aslan is talking about is "Jesus Christ."
Aslan also appears in the book in several other symbolic animal forms that are connected with Christ and the Christian trinity. In the final chapter, Aslan begins by taking the form of a lamb, a reference to Christ as the "Lamb of God," and offers the children a meal of fish, something Jesus and his followers eat frequently (and that Jesus is able to perform miracles with) in the New Testament. During the episode of the Dark Island, Aslan leads the Dawn Treader away from harm in the form of an albatross that at first "looked like a cross," reminding readers not only that the albatross is a bird that leads sailors to safety in maritime legends, but also that the Holy Spirit is sometimes represented as a white dove. Don't make us explain what's symbolically Christian about it looking like a cross…we think you can figure out that one on your own!