The spirituality of the Legion of Mary is essentially based on the approach of St. Louis-Marie Grignon de Montfort as put forward in his book True Devotion to Mary. Grignion de Montfort promoted a "total dedication" to Christ through devotion to the Blessed Virgin Mary, which later also influenced popes such as John Paul II, as mentioned in his Apostolic Letter Rosarium Virginis Mariae.
An important element that shaped Legion spirituality was Frank Duff's devotion to the Holy Spirit. He promoted the (in popular devotion often neglected) adoration of the Third Person of the Trinity through the Virgin Mary, Whom he saw as the "visible image" of the Spirit. This is why the introductory prayers and the Legion promise are directed to the Holy Spirit and the vexillium Legionis bears His image in the form of a dove.
The essential aim of the Legion of Mary is the sanctification of its members through prayer, the sacraments and devotion to Mary and the Trinity, and of the whole world through the apostolate of the Legion.
The idea of an organization where ordinary laypeople in all situations of life would work for their own sanctification and for the conversion of the world was groundbreaking for its time. Only when the Second Vatican Council (1962–65) promoted such ideas in its documents did such an approach gain wider acceptance in the Catholic Church.