From the earliest centuries, “Christ’s open side and the mystery of blood and water were meditated upon, and the Church was beheld issuing from the side of Jesus, as Eve came forth from the side of Adam. . . . It is in the eleventh and twelfth centuries that we find the first unmistakable indications of devotion to the Sacred Heart. Through the wound in the side, the wounded Heart was gradually reached, and the wound in the Heart symbolized the wound of love.” (from the Article “Devotion to the Sacred Heart of Jesus” in the Catholic Encyclopedia) But the devotion became even more widely popularized when St. Margaret Mary Alacoque (1647-1690), a Visitation nun, had a personal revelation involving a series of visions of Our Lord as she prayed before the Blessed Sacrament.
In these visions Our Lord promised
A general devotion to the Sacred Heart was popular in monasteries in the Middle Ages, especially in response to the devotion of St. Gertrude the Great (b. 1256).

St. Gertrude the Great

St. Margaret Mary
1
He will give them all the graces necessary in their state of life.
2
He will comfort them in all their afflictions.
3
He will establish peace in their homes.
4
He will be their secure refuge during life, and above all, in death.
5
He will bestow abundant blessings upon all their undertakings.
6
Sinners will find in His Heart the source and infinite ocean of mercy.
7
Lukewarm souls shall become fervent.
8
Fervent souls shall quickly mount to high perfection.
9
He will bless every place in which an image of His Heart is exposed and honored.
10
He will give to priests the gift of touching the most hardened hearts.
11
Those who shall promote this devotion shall have their names written in His Heart.
12
In the excessive mercy of His Heart, His all-powerful love will grant to all those who receive Holy Communion on the First Fridays in nine consecutive months the grace of final perseverance; they shall not die in His disgrace, nor without receiving their sacraments. His divine Heart shall be their safe refuge in this last moment.

St. Margeret Mary
Devotions that are attached to these promises include:

St. Margaret Mary