Law's practical work as a spiritual director—as expressed in A Serious Call to a Devout and Holy Life—deeply influenced the English Evangelical Revival. The Oxford Dictionary of the Christian Church says of the book: "The simplicity of its teaching and its vigorous style soon established the work as a classic, which has probably had more influence than any other post-Reformation spiritual book except Bunyan's Pilgrim's Progress." In his later years Law became an intense admirer of the teaching of Jacob Boehme on the coincidence of opposites. He gave this doctrine an original turn in his little-known but exquisite mystical treatises—the most important being The Spirit of Love.