Free agency
The idea of free agency is present throughout Twilight. Bella, for her part, seems utterly compelled to stay with Edward, no matter who or what he is, so much so that it seems like she lacks any free agency. She does frame it as a decision, so we know at least that she believes there was an alternative, whether she felt free to choose it or not.
Carlisle and Edward represent the other side of the spectrum. Carlisle especially, from the very beginning, has been able to overcome his strongest instincts, to rise above the killer that he has become. Edward, when he first meets Bella, does not think he will have the power to choose not to kill her, but in the end, he makes this decision and sticks to it. Alice’s powers also highlight this idea, for she can only know the future so far as people’s decisions have been made, and as they change, so too does the future. Thus people’s decisions do affect their futures.
Fate
The theme of fate is bound with the theme of free agency. When Bella believes she is about to die, she looks back on her decision to come to Forks as the moment that led her to where she is. She cannot regret her choice, subconsciously showing her respect for fate. There were many smaller moments after that decision that could have led her on another path—most simply, if she had not gone to the Cullens’s baseball game. Yet for her, from the moment she decided to come to Forks, everything else felt fated. Fate is thus something more for her than the playing out of one's decisions.
She makes this clear, also, when she describes herself as never really having had a choice of whether to stay with Edward or not. The depth of her love for him has never felt like a choice; it has felt like something fated. Edward’s feelings for her also seem to be on the level of inevitability, yet he has two competing desires, and it is his free agency that allows him to choose loving Bella over killing her. Meanwhile, they both joke about how she seems fated to die soon--sooner than the inevitable death of a mortal human being--presuming that she never becomes a vampire.
Self-control
Self-control in the face of temptation is a very important theme in Twilight, and it is closely tied to the theme of free agency. In order to exert agency, the vampires must have incredible self-control. Carlisle is the epitome of this dynamic, being capable even of performing surgery on humans without letting the blood attract him too strongly. Self-control is then the key to overcoming destiny or instinct, the way to maintain free agency and the life and principles one wants to have.
Edward, too, is able to exert incredible self-control. He is capable of being close to Bella although her scent is overwhelmingly attractive to him. Bella, on the other hand, does not seem quite so skilled at self-control. Every time Edward comes close to her, she loses some level of control, often to an extreme, even when she knows that her life could be at stake.
The book's epigraph refers to a verse in Genesis regarding "the tree of the knowledge of good and evil." The apples on this tree are a great temptation, and when Eve gives the apple to Adam, the scene also suggests that woman is a temptation to man. This opening reference suggests that Bella is a serious temptation for Edward. In act, when Bella says, "He didn't know me from Eve," in the first chapter, readers understand that soon enough Edward will be playing Adam to Bella's Eve.
Sacrifice
In order for Bella and Edward to ultimately be together, Bella must sacrifice her life and her family. Although Bella feels ready to make this profound sacrifice, Edward is not ready to accept it. Sacrifice is certainly a loss, but it can be acceptable or even noble when it is a free choice to gain something even better. Is it enough in this case, to do it for someone she loves? It is harder to accept when someone else has to make the sacrifice for one's own sake.
In the end, Bella chooses to sacrifice her life to save her mother, Charlie, and hopefully the Cullens. As long as she believes that this sacrifice will have meaning, she can face death with more bravery than she otherwise would. Edward, however, again is not able to let her make that sacrifice. It also seems unlikely that, if she had died, he would have let James get away with it, as she hoped.
Good and evil
What makes a person, or a vampire, good or evil is an important theme in Twilight, and it is closely connected to the themes of self-control and free agency. Although most vampires hunt humans, in Twilight this habit does not make them inherently evil. They are (super)natural predators, not killers. Even so, however, they cannot truly be good until they have exerted their free agency, and made the choice, using their self-control, to rise above their nature.
It is vampires like James who are truly evil, taking pleasure in the hunt--even in torturing his prey both physically and psychologically. It is thus not what you are born as, or what you are made to be, that defines whether you are good or evil, but what choices you make on the basis of your identity and your powers. Self-control, the willingness to sacrifice part of oneself for a better purpose, leads to good results in Twilight.
Time and eternity
The contrast between Edward’s immortality and Bella’s mortality makes time and eternity into interesting themes in Twilight. For Edward, time moves slowly because he never changes physically, and he has seen so many years pass that the small changes become less noticeable. For Bella, however, time seems to fly by, and she fears that every moment ages her and thus takes her farther from Edward.
Her impending death, while hopefully a long way off, feels to her like it will come at any moment, for Edward will still have an eternity after she is gone, and if he would only change her, she could have an eternity with him. A human lifespan in comparison to eternity seems very, very short. Throughout the novel this feeling pervades Bella’s moments with Edward; she always feels that they are rushing by too quickly, that they are fleeting and passing by forever so that she will never be able to have the same thing again.
Faith
Faith is a rather subtle theme in Twilight, but it is an important one. The only time when it is explicitly mentioned in relation to religion is when Edward discusses how vampires originally came to be--whether they evolved or were created alongside their human prey. Edward finds it hard to believe that there is no greater purpose to the world, and that is the source of his faith.
But faith in this novel is not just about religion. Bella has ultimate faith in her love for Edward and in his for her, such that she is willing to risk her life to be alone with him--to kiss him--and she is willing to give up her family and her life to be with him forever. She also has faith in his goodness, even knowing that he has hunted humans before and that when he was first with her, he was not sure that he would be able to prevent himself from killing her. Faith involves elements of trust and hope, perhaps tinged with fear.
