The Brown Scapular represents the Mantle of Mary, and wearing it is a way of showing that you are under her protection.
Asking for Mary’s protection is a common tradition in the Church around the world. In fact, one of the oldest prayers of the Church is the Sub Tuum Praesidium (“Under Your Protection”).
Sub Tuum Praesidium:
Sub tuum praesidium confugimus,
Sancta Dei Genetrix.
Nostras deprecationes ne despicias in necessitatibus,
sed a periculis cunctis libera nos semper,
Virgo gloriosa et benedicta.
We fly to thy patronage,
O holy Mother of God;
despise not our petitions in our necessities,
but deliver us always from all dangers,
O glorious and blessed Virgin.
From the Legion Handbook:
Legionaries will not neglect the use of the scapulars, medals, and badges approved by the Church. In distributing these and spreading devotion to them, channels are set up, along which — as a million instances have shown — it is the will of God that grace will copiously flow.
Particularly, they should think in terms of the brown scapular which is the very livery of Mary. “Some interpret literally the text: ‘He who dies wearing this habit will not be lost.’ St. Claude de la Colombière would brook no restriction: ‘One may lose one’s scapular, but one who wears it at the hour of death is saved.’ ” (Père Raoul Plus)Legion of Mary Handbook , Ch 39, #34 , pg 305-306
More on the Scapular Story:
The most highly developed of Marian Scapular devotions is that of the Brown Scapular of Our Lady of Mount Carmel. Since the seventeenth century, the Brown Scapular has been a universal Catholic devotion, considered to be, together with the rosary, a customary form of Marian devotional practice.
The wearing of the Scapular fosters a true devotion to Mary that is based on her supernatural mission in the redemption of mankind. Two Marian doctrines are proposed in the devotion of the Brown Scapular: Mary’s Spiritual Maternity and her Mediation of Grace. The Scapular teaches a practical confidence in the intercession of the Blessed Virgin to obtain for its wearer the grace of final perseverance, or a happy death. The two general conditions to obtain this benefit are that one must honor Mary by wearing the Scapular faithfully until death and endeavor sincerely to lead a Christian life. This reliance on Mary’s intercession for the gift of final perseverance derives historically from the belief that the Blessed Virgin promised in an apparition to St. Simon Stock, Prior General of the Carmelites (1247?-1265), that all who die wearing the Scapular will not suffer the eternal flames of hell. This tradition has become known as the “Scapular promise.”
The devotion also teaches that the aid of Mary may be confidently expected in purgatory by all those who have faithfully worn the Scapular and have fulfilled two other conditions: the practice of chastity according to one’s state of life and the daily recitation of the Little Office of the Blessed Virgin. This privilege of the Scapular devotion has been thought to stem from an apparition of Mary to Pope John XXII, who then promulgated this spiritual benefit to the faithful in 1322. According to the copies of the Bull of promulgation attributed to John XXII, the devotee of the Scapular would be released from purgatory on the Saturday after death. Because of the allusion to Saturday, the document of John XXII has been called the “Sabbatine Bull” and its Marian privilege the “Sabbatine Privilege.”
THE SCAPULAR DEVOTION IN MODERN LIFE
The popes in modern times have been solicitous in their encouragement of the Scapular devotion. St. Pius X permitted the substitution of a Scapular Medal for the cloth Scapular in recognition of the changed circumstances of life, precisely to encourage the dedication to Mary signified by the Scapular. The Medal was not intended as a new form of the Scapular devotion, but only as an aid to its continual practice. Catholics should be instructed to make free and wise use of both Scapular and Medal according to their judgment and circumstances. The permission for the Medal reflects the mind of the Church that the Scapular itself is only the exterior sign of an interior devotion.“The Scapular Devotion” by Christin P. Ceroke, O.Carm.