This is a novel of good versus evil, and the few good people who understand that the vampire Count Dracula has come to England in a first step towards world conquest for the evil one know this is a Christian struggle of God against the devil and light against darkness.
From the start, Dracula and his legions of the undead are associated with darkness. Dracula only emerges at night—he sleeps, as Harker discovers, in a coffin by day. The light of a mirror doesn't reflect his image, and he is associated with and commands creatures traditionally connected to the devil and darkness, such as rats, wolves, and bats.
The connection of the undead to evil and soulless devils ("fiends") is established early on. When Harker encounters the vampire women in Dracula's castle, he describes them as follows:
On this the other women joined, and such a mirthless, hard, soulless laughter rang through the room that it almost made me faint to hear; it seemed like the pleasure of fiends.
Van Helsing is a remarkable scientist, but he is also a Christian and will later remark: "How are all the powers of the devils against us!”
Van Helsing explains that Lucy's fate, like all who are bitten by vampires, is to lose her Christian soul. To free any human from the undead is to liberate their souls from evil. Death is far preferable to being in the liminal, dark state of vampirism, where one is at the command of the devil. He says of Lucy:
But of the most blessed of all, when this now Un-Dead be made to rest as true dead, then the soul of the poor lady whom we love shall again be free. Instead of working wickedness by night and growing more debased in the assimilating of it by day, she shall take her place with the other Angels.
Up until her encounter with the vampires, Lucy has been chaste and Christian, a picture of purity, as is Mina, and all the other characters determined to stop Dracula. There are too many fighting Dracula to enumerate the Christian qualities of each one, but each believes in the supernatural quality of the quest to end the spread of vampirism. Each one is willing to act in a traditionally Christian manner and sacrifice themselves for the good of others and of the world. All believe that salvation of the eternal soul is preferable to mortal life itself. This adds to the urgency of their mission: they are fighting not only for the bodies but for the souls of all humans.