John Wesley said that of all the definitions of Christianity that he had encountered, the best was that of a Scotsman who lived in the 17th-century. He said: "Christianity is the life of God in the soul of man."

This timeless classic was originally written to encourage a friend and stimulate his spiritual life. It was so appreciated that it was later published as a book for a wider readership.

100 years later a copy was sent to George Whitefield by his friend Charles Wesley, it was instrumental in Whitefield's conversion. This book provided much of the stimulation behind the Methodist Revival of Britain and the Great Awakening in America.