This is a reference to John Bunyan's The Pilgrim's Progress, a 16th century allegorical story about the journey through the Christian life. Much persecuted for his faith, John Bunyan spent many years in prison and wrote several radical texts about the experience of believing in God. The Pilgrim's Progress is the most famous. It has never been out of print.
It follows the adventures of Christian as he travels through a range of weird and wonderful landscapes and places, including Vanity Fair, House Beautiful and the Slough of Despond. He is helped on his journey by the Evangelist, who explains some of the mysterious things he sees and encourages him when he is losing heart. At one point, when Christian is feeling faint, the Evangelist refreshes him at the River of Life.