In the present work, Law examines the principles of Christian devotion. In so doing, he places tremendous emphasis on prayer. Concerning this work, John Wesley commented, "Meeting now with Mr. Law's 'Christian Perfection' and 'Serious Call,' although I was much offended at many parts of both, yet they convinced me more than ever of the exceeding height and breadth and depth of the law of God. The light flowed in so mightily upon my soul that everything appeared in a new light . . . I was convinced more than ever of the impossibility of being half a Christian."
Dr. Samuel Johnson referred to Serious Call as "the finest piece of hortatory theology in any language." He also shared, "When at Oxford, I took it up expecting to find a dull book, and perhaps to laugh at it. But I found Law quite an over-match for me; and this was the first occasion of my thinking in earnest of religion after I became capable of religious inquiry."
What is interesting about these endorsements is that both Wesley and Johnson had, elsewhere, been just critics of William Law, especially Law's association with Behmenism.