This personality article is an opinion by a fan and should not be considered canon.
The Christian significance of Susan's character has been much discussed. Aside from her role together with Lucy paralleling the women in the gospel who first find the risen King, Lewis may have intended her to represent the good seeds which are "choked by thorns" in the parable of the sower from the Gospel of Matthew. It is also quite likely, that Lewis may have intended to keep Susan for a post-Narnia story, about redemption, but died before writing it, as he had written to his young fans that Susan's story was not finished.
In a letter written to a young fan, C. S. Lewis stated that Susan's story was not finished.
Lady Polly claims that Susan's "whole idea is to race on to the silliest time of one's life as quick as she can and then stop there as long as she can." That is, Susan's failure is due to vanity and a false adolescent sense of "maturity". This image of Susan provides a striking contrast to her sister Lucy, who is a shining example of the Biblical "faith as a little child."
It has been argued that Susan's maternal nature cultivates a sense of self-reliance that prevents her from sufficiently following Aslan. In this interpretation, Lewis intended Susan to represent those who in the confusion of their fallen state find a spiritual call to faith drowned out not by malice on their part but simply by the mundane distractions of everyday life.
It can be argued that in his portrayal of Susan, Lewis is attempting to illustrate the importance of keeping important things in focus, for by devoting her entire present life to something temporary, Susan sacrifices her chance at something eternal. Lewis is not stating that Susan's natural maturing is inherently wrong, but to become overly devoted to petty and shallow aspects of it is.
There remains controversy among fans as to whether Susan's absence in Aslan's Country was permanent. It is possible that once she remembers Narnia as it was and remembers her place in it as Queen, she will be able to go to Aslan's Country when she dies.
It can also be pointed out that the other children enter into the "new" Narnia (representative of the eternal Heaven) because they have died in a train accident, while Susan remains alive on our world, providing no proof that she has been permanently excluded. Aslan's last words at the coronation of the four Pevensies offer the best justification for believing Susan will eventually join the others when the time comes in Aslan's Country: "Once a King or Queen of Narnia, always a King or Queen of Narnia."
Another thing that should be noted was that at the Pevensie's coronation, was Aslan saying "may your wisdom grace us until the stars rain down from the Heavens". However, since Narnia had already been destroyed and "the stars rain down" before Susan died, this may mean that Susan has missed her chance, but since many of Susan's friends from the old Narnia, are in the "new" Narnia, and everything was "the same, but better", with the "new" Narnia having night, the stars are in "the Heavens" there, which could mean that Susan will join the others when she dies.
Lewis once said that Susan's journey was not finished. Perhaps he saw saving Susan for something more, maybe he was giving her a second chance, since Susan had "listened to fears" before. Also Susan must have greatly suffered when told that her parents, siblings, cousin and friends had all been killed. Maybe upon learning of this, Susan would begin to see life more clearly, and become like she had been as a Queen of Narnia, foresaking her silliness and embracing life again, and prepared to accept disappointment, finally joining the others in the "new" Narnia when she dies.