You already have one great answer, and I agree that both Tolkien and L'Engle use religion as the basis for their fiction work. In addition to the ideas presented by herappleness, I would add the rather simple observation that both authors clearly reflect the battle between the forces of light and the forces of darkness and the eventual triumph of good over evil. While one uses human characters in otherworldly settings (as in L'Engle's Starfishtrilogy) and the other uses creatures in a world of make-believe (as in Tolkien's Lord of the Rings trilogy), both clearly outline a battle in which evil is real but is a conquered foe. This is the exact message of traditional religious belief and a consistent motif used by these two authors.
Well, yes, indeed it is religion. Both Tolkien, his friend CS Lewis (while they would be referred to as the 'inklings'), and Madeline L'Engle were pioneers and creators of Christian Mytheopeia, that is, imagery, stories, themes, and literary works dedicated to the intricate relationship between man and religion, man and spirituality, the spirit and the body, the terrestrial and the ethereal. Included to this list are Bunyan, Alighieri, and any other author whose literary technique is to infuse the magical with the terrestrial in hopes of creating new and fascinating reading experiences. These great writers are being studied in modern literature and, as you can see, from CS Lewis, to Tolkien's Two Towers, it seems as if literature always has a special place for modern mythology (namely Christian mythopeaia) as an alternative to realistic fiction or non fiction. It is a favorite among young readers, and it seems to withstand the changes of time!