Few would argue with the claim that John Bunyan's most important contribution to English literature is Pilgrim's Progress. At one point it was second only to the bible in terms of sales and remains one of the bestselling books of all time.

It is popular for many reasons. First, Bunyan was bought up poor and mostly uneducated. So when he did learn to read and write, he wrote with a simple everyman style that everyone from children to tradesman could identify with.

Second, he wrote the book during a time that England was trying to rid itself of the corruption that had become an identified with the Catholic Church and move towards the more morally pure form of Christianity embodied by the Puritans. Pilgrim's Progress is not only an allegory of that transformation but is probably the most famous book by a Puritan writer.

Remember also that, at that time in history, many Puritans had settled in America. For the first time, people were reading English language books outside of Europe.

Finally, John Bunyan suffered at the hands of the English Church, who he felt were still holding onto the power of the Catholic Church. He wrote the first part of Pilgrim's Progress from prison. Six years after its publication, England had the Glorious Revolution where they shook off the constraints of Catholicism. From this perspective, you can imagine that the protestants celebrated Bunyan's work as an expression of their new world.