On May 25, 1671, at the age of 23, Margaret entered the Order of the Visitation convent at Paray-le-Monial. She pronounced her final vows on November 6, 1672 and took the name Mary. During her retreat before her profession, she had a vision of Jesus in which He said, “Behold the wound in my side, wherein you are to make your abode, now and forever.” The Lord continued to appear to her in visions and on December 27, 1673, the feast of Saint John the Evangelist, as she knelt at the grill before the exposed Blessed Sacrament, she experienced a vision in which the Lord told her to take the place that Saint John had occupied at the Last Supper, and that she would act as His instrument. Jesus revealed His Sacred Heart as a symbol of His love for mankind, saying, “My divine Heart is so inflamed with love for mankind … that it can no longer contain within itself the flames of its burning charity and must spread them abroad by your means.” She described that His Heart was on fire and surrounded by a crown of thorns. Our Lord told her that the flames represented His love for humanity, and the thorns represented man’s sinfulness and ingratitude. Jesus informed her that her mission was to establish the devotion to His Most Sacred Heart, and He revealed twelve promises that He would bestow upon all those who practice the devotion. read more…
source: http://www.sacredheartapostolate.com/MargaretMary.html