What is the theme in The Pilgrim's Progress by John Bunyan?

The pilgrims seeking salvation follow a strict Calvinist religious philosophy. This philosophy argues that no one reaches Heaven through the systematic practice of religion or the general good nature of his or her character. Instead, people are specifically chosen by God and are marked by him in order to gain admission to Heaven. All other paths are useless in attaining salvation and God must bless one personally, usually with visions or revelations, in order to receive entry, according to this philosophy.