Parley P. Pratt began to work on The Key to the Science of Theology in San Francisco in August 1851, just before leaving for his mission to Chile. He continued writing it while in Chile. He finished the manuscript upon his return to Utah in 1853. The manuscript remained unpublished, however, until 1855 when Parley found a publisher in England. Parley's book represented a lifetime of thought and built on ideas first propounded in Pratt's earlier writings, especially missionary tracts stemming from his years in England.
The impact of The Key to the Science of Theology was unparalleled. One Latter-day Saint diarist of the 19th century observed: "We had every encouragement to read the Church publications: The Voice of Warning; the Pearl of Great Price; and Key to Theology."
That Parley's writings would be on the same footing as the Pearl of Great Price, canonized in 1880, indicates the special status his works held among early Church leaders and members.
During the 22 years following its publication, a time when almost no other Church books were being written, The Key to the Science of Theology went through three more editions, suggesting that the work had the tacit approval of Brigham Young.
Indeed, in 1875, Voice of Warning and Key to the Science of Theology were the first two titles mentioned in the Church's Deseret News list of "Books worth Reading".
Parley's Key to the Science of Theology continues to be a book worth the time today, providing the modern listener insight into the early gospel thinking of Church leaders.
Great insights about Parley Pratt's understanding of the gospel. My favorite quote about the Holy Ghost is found below:
""It quickens all the intellectual faculties, increases, enlarges, expands and purifies all the natural passions and affections, and adapts them, by the gift of wisdom to their lawful use. It inspires, develops, cultivates and matures all the fine toned sympathies, joys, tasts, kindred feelings and affections of our nature. It inspires virtue, kindness, goodness, tenderness, gentleness and charity. It develops beauty of person, form and features. It tends to health, vigor, animation and social feeling. It develops and invigorates all the faculties of the physical and intellectual man. It strengthens, invigorates and gives tone to the nerves. In short, it is, as it were marrow to the bone, joy to the heart, light to the eyes, music to the ears, and life to the whole being."