The Pilgrim's Progress represents the rarest of achievements, a nearly flawless work of art created for nonartistic ends. Bunyan cast his book in the form of a religious allegory; but it is allegory transformed into intense drama, its style an unsurpassed evocation of the power of plain English prose. The multitude of characters who appear on its pages escape the bounds of narrow didactic design; they are superbly individualised, indelibly alive, as memorable as are those landmarks on Christian's perilous journey toward salvation: the Slough of Despond and the Delectable Mountains, Doubting Castle and the Palace Beautiful, Vanity Fair and the Celestial City. The author's religious faith, pure and ever present, serves to imbue the immediacy of his narrative with added dimension; the result belongs to the highest realm of literature, a continuously compelling tale framed in an unwavering vision of eternity. As F. R. Leavis writes: "The Pilgrim's Progress stands alone... the work of a great creative writer... there was only one Bunyan."