1) The devotion to the Sacred Heart is over 900 years old: In the 11th century St. Peter Damien wrote the following: “It is in the adorable Heart of Jesus that we find every weapon proper to our defense, every remedy for the cure of our ills, the most powerful assistance against the assaults of our enemies, the sweetest consolation to relieve our sufferings, the purest delights to crown our souls with Joy.”
2) There was more than one saint who had visions of the Sacred Heart: In the 1200's three nuns started the monastery of St. Mary of Helfta. The nuns were St. Mechtild of Magdeburg, St. Mechtild of Hakeborn and St. Gertrude the Great.
3) Doesn't everyone have visions? St. Margaret Mary started having visions of Jesus, typically crucified, early in her life. She assumed that this was normal for everyone.
4) The Feast of the Sacred Heart was celebrated before St. Margaret Mary's visions: On August 31, 1670 a celebration in honor of the Sacred Heart was celebrated at the seminary in Rennes.
5) St. Margaret Mary Alacoque had multiple visions of the Sacred Heart: From 1673 – 1675. During the visions Christ told St. Margaret that he wanted three specific devotions done to honor His Sacred Heart – frequent communion, receiving Communion at Mass on the first Friday of the month and regular holy hours.
6) The Jesuits were involved: St. Margaret Mary's confessor was St. Claude de Colombiere. In 1675 Jesus asked St. Margaret to tell Fr. Colombiere about her visions and entrusted the propagation of the devotion to the Sacred Heart to the Sisters of the Visitation and the Society of Jesus. Fr. Colombiere asked St. Margaret to write an account of the visions, consecrated himself to the Sacred Heart and spent the rest of his life spreading information about the devotion in France and England.
7) The Church didn't move quickly: While the Church official granted St. Margaret's order, the Visitandines, permission to celebrate a Mass in honor of the Five Wounds of Christ in 1697, it wasn't until 1856 that Pope Pius IX made the Feast of the Sacred Heart a universal celebration. In 1899 Pope Leo XIII consecrated all mankind to the Sacred Heart.
8) The devotions to the Sacred Heart and the Divine Mercy are closely linked: During his homily at the first universally celebrated Divine Mercy Sunday, Bl. Pope John Paul II said
His “Sacred Heart” has given men everything: redemption, salvation, sanctification. Saint Faustina Kowalska saw coming from this Heart that was overflowing with generous love, two rays of light which illuminated the world.
The two rays, [according to what Jesus Himself told her], denote blood and water (Diary, 299). The blood recalls the sacrifice of Golgotha and the mystery of the Eucharist; the water, according to the rich symbolism of the Evangelist John, makes us think of Baptism and the Gift of the Holy Spirit (See Jn 3:5; 4:14).
Through the mystery of this wounded Heart, the restorative tide of God's merciful love continues to spread over the men and women of our time. Here alone can those who long for true and lasting happiness find its secret.”
9) Jesus was very serious about the mercy He offered through devotion to His Sacred Heart: Jesus made several promises to St. Margaret Mary for those who received communion in a state of grace on nine consecutive first Fridays but the most important one was
“I promise the Grace of final perseverance. They will not die in My disgrace, but will receive the Sacraments (if necessary), and My Heart will be sure shelter for them in that extreme moment.”