One of the most honourable titles given to St Louis Marie ("the priest with the big rosary") is that of "Apostle of the Cross and of the Holy Rosary". The Rosary occupied an important place in his own spiritual life and in his apostolate. The Secret of the Holy Rosary (as it is sometimes called) is not as original in its composition as his other books, for here he is demonstrating the value of one particular devotional practice (among others mentioned in the True Devotion to the Blessed Virgin), and at the same time borrowing extensively from many authors.
As he was primarily a missionary of the ordinary people, concentrating especially on the poor and abandoned, he set out to renew in them the spirit of Christianity, believing that this could be achieved by devotion to Mary which could only lead people to Jesus and holiness. He believed that the Rosary was a wonderfully secret way of knowing Mary and finding Jesus through her. He established the devotion of the Rosary wherever he preached and had it recited publicly every day during his missions. This book, though not published in his lifetime, was certainly intended for all classes of people, as is clear from the "Little Roses" in the introduction. In the body of the book, which (like the Rosary itself) is divided into Decades, each with ten "roses", he speaks about the origin of this form of Marian devotion and the climate of miracles in which it developed over the centuries. Well aware that critics would express doubts about some of his stories, he simply points out that he has quoted them from reputable authors. Other sections of the book deal with the power and effectiveness of the Rosary, the prayers of which it is made up, the beauty and usefulness of the meditations which should accompany it. He describes how to say the Rosary "worthily", and at the end provides some Methods of Reciting the Rosary.