A prominent woman in Parisian society, Marie de la Guesle-the Countess of Châteauvieux-met Mother Mectilde of the Blessed Sacrament on a charity visit in August 1651. So taken was she with the Prioress's wisdom that she returned several days later to resume the conversation. Soon the nun began to serve as the Countess's spiritual guide, leading her in the paths of faith, self-renunciation, prayer, and the pure love of God. The Countess and her husband became great benefactors of Mother Mectilde's newly established order, the Benedictines of Perpetual Adoration of the Most Holy Sacrament, and on the day after her husband's death in November of 1662, the Countess herself entered the convent she had helped to build.

A permanent record of the friendship, which grew ever deeper between the mystic of Lorraine and the grand lady, was left in their candid and multifaceted correspondence, conducted over a period of about a dozen years. Marie later arranged Mectilde's letters into a volume which she called her "breviary of fire." Benedictine in its fundamental principles, Mother Mectilde's teaching reflects and contributes to the French School, which included such luminaries as Pierre de Bérulle, St. John Eudes, and Jean-Jacques Olier, while drawing freely on other sources, especially St. John of the Cross and St. Francis de Sales. Mother Mectilde's gifts of psychological penetration and practical mysticism are fully evident in the advice she gave to Marie de Châteauvieux, which her daughters in religion eagerly recopied and passed down for over three centuries, and which is now available for the first time in English in a translation that faithfully reflects the fiery quality of the original.