Margaret Alacoque (1647-1690) was born in Verosvre (France). She became a Visitation nun in Paray-le-Monial (1672) and later Novice Mistress.
Margaret Mary and the Virgin Mary
During her childhood, Margaret was healed from a serious four-year illness through Mary's intercession. In thanksgiving, she added the name of “Mary” to her baptismal name on the day of her confirmation.
"I was going to her with so much confidence that I felt I had nothing to fear under her maternal protection. I devoted myself to her to be forever her slave, begging her not to refuse me in this quality. I spoke to her like a child, with simplicity, just like my good Mother, for whom I felt inclined with loving affection from then on. I entered the Visitation because I was attracted by the very sweet name of Mary. I felt that was what I was looking for."
Once in the convent, she fell ill, and again it was the Virgin Mary who healed her: she appeared to Margaret Mary, caressed her, and said: "Take courage, my dear daughter, in the health that I give you from my divine Son, for you have yet another long and painful journey to make, always on the cross, pierced with nails and thorns, and torn with whips; but fear nothing, I will not abandon you and promise you my protection."
Devotion to the Sacred Heart already existed 
Devotion to the Sacred Heart was already practiced by St Anthony of Padua, St Bonaventure, and S. Clare of Assisi in the 12th – 13th centuries, and in St Margaret Mary Alacoque’s own time, by Berulle (Catholic priest, cardinal and statesman, one of the most important mystics of the 17th century in France) and St John Eudes. Images of Jesus showing his heart in his half-open chest already existed before the apparition.
The central idea of the devotion to the Sacred Heart can be summed up as: "What a joy to be united to Jesus Christ in the Sacred Heart who has been continually united to God."
For some people (including Margaret Mary), devotion to the Sacred Heart is about praying for sinners, in reparation for sins.
A new impetus for the devotion to the Sacred Heart
Sister Margaret Mary evoked several apparitions of Christ:
- On December 27, 1673, feast of St John the Evangelist, she had a little extra free time and chose to spend it in prayer before the Blessed Sacrament: “Jesus said to me, ‘My divine heart is so passionately in love with men, and you in particular, that, being unable to contain the flames of its ardent charity, it must spread them with your help, so it will visible to them and enrich them with its precious treasures, which I am revealing to you, as they contain the sanctifying and salutary graces necessary to keep them from falling into the abyss of perdition. I have chosen you as an abyss of unworthiness and ignorance for the fulfillment of this great purpose, so that all things will be done by me.’
Afterward, he asked me for my heart, which I begged him to take. He did, and put it in his own adorable Heart. Inside it, He made me see my heart as a little atom which was consumed in that burning furnace. After he drew it out like an ardent flame in the shape of a heart, he put it back in its place, saying to me: ‘Behold, my beloved, a precious pledge of my love, which will enclose in your side a little spark of its bright flames, to serve you as a heart and consume you until the last moment ... I have a burning thirst to be honored by men in the Blessed Sacrament, and I find almost no one who strives, according to my desire, to quench my thirst, returning my love.’ " 
- On the first Friday of a month in 1674, Jesus asked for an act of reparation for the offenses against the Blessed Sacrament, by offering the hour (Thursday from 11pm to midnight) and communion of the first Friday of the month.
- One day in the octave of the feast of the Blessed Sacrament (Corpus Christi) in 1675, Jesus asked for the institution of an annual feast of the Sacred Heart. 
- One day in the year 1689, Jesus told Margaret Mary that he wanted the king, Louis XIV, to consecrate himself to the Sacred Heart, to add the image of the Sacred Heart on the French flag, and to build a national shrine dedicated to the Sacred Heart in which he would consecrate France to the Sacred Heart of Jesus. But Louis XIV didn’t heed these wishes, and the Sacre-Coeur Basilica in Montmartre would only be inaugurated in 1919.
We should remember that by 1688, Louis XIV had brought the royal power and prestige of France to their culminating point. After 1688, misfortunes started befalling the kingdom and the reign of Louis XIV has been the object of many criticisms. It was during the pivotal years of 1688-1690 that Our Lord, through Margaret Mary, called King Louis XIV the "Elder Son of His Sacred Heart" and his "Faithful Friend." Sadly, this great king did not deign to take the conditional promises of the King of Heaven into consideration.
 Marquis de la Franquerie, La Vierge Marie dans l'Histoire de France, Editions saint Rémi, p. 174
 Cf. E.Préclin et E.Jarry, Histoire de l'Eglise, vol. 19 , Bloud & Gay, Paris 1955, p. 288-289.
Recommended read also: H. De Barenton, La dévotion au Sacré-Coeur. Ce qu'elle est et comment les saints la pratiquèrent, Paris 1914. L. Garriguet, Le Sacré-coeur de Jésus. Exposé historique et dogmatique de la dévotion au Sacré Coeur de Jésus, Paris 1920.
 Cf. http://www.abbaye-saint-benoit.ch/saints/margueritemarie. Citation de l'Autobiographie, p. 75.
 Cf. E. Préclin et E. Jarry, Histoire de l'Eglise, vol. 19 , Bloud & Gay, Paris 1955, p. 288-289