The Catechism of the Catholic Church (p.478), states, "Jesus has loved us all with a human heart. For this reason, the Sacred Heart of Jesus, pierced by our sins and for our salvation, 'is quite rightly considered the chief sign and symbol of that... love with which the divine Redeemer continually loves the eternal Father and all human beings' without exception.”
To appreciate this rich symbolism of the heart, we must remember in the Old Testament the word heart represented the core of the person. While recognized as the principle life organ, the heart was also considered the center of all spiritual activity. Here was the seat of all emotion; especially love. The heart has even greater depth when contemplated in light of the Incarnation. We believe that Jesus Christ, second person of the Holy Trinity and consubstantial with the Father, entered this world taking on our human flesh. His Sacred Heart represents love: the divine love our Lord shares with the Father and Holy Spirit in the Trinity; the perfect, divine love which God has for us; and the genuine human love Christ felt in His human nature. We hear our Lord saying "Come to Me, all you who are weary and find life burdensome, and I will refresh you. Take My yoke upon your shoulders and learn from Me, for I am gentle and humble of heart. Your souls will rest, for My yoke is easy and My burden light" (Mt 11:28-30). Therefore, while meditating on the Sacred Heart of Jesus, we are called to share in the love of the Lord and strive to express our own genuine love for God, ourselves and our neighbors.
Throughout the Gospels, we see the outpouring of Jesus' love from His heart, whether in miracle stories, the reconciliation of sinners, or the compassion for the grieving. Even on the cross, our Lord poured out His love for us: there the soldier's lance pierced His side and out flowed blood and water (Jn 19:34). St. Bonaventure said the Church was “…born from the wounded side of the Lord with the blood and water representing the Sacraments of the Holy Eucharist and baptism.” The early Church Fathers clearly cherished this meaning of the Sacred Heart of our Lord. St. Justin Martyr (d. 165), in his said, "We the Christians are the true Israel which springs from Christ, for we are carved out of His heart as from a rock." Likewise, St. Irenaeus of Lyons (d. 202) said, "The Church is the fountain of the living water that flows to us from the Heart of Christ.” St. Paulinus of Nola (d. 431) added, "John, who rested blissfully on the breast of our Lord, was inebriated with the Holy Spirit, from the Heart of all creating Wisdom he drank an understanding which transcends that of any creature." Although these are just a few brief examples from the times of the early Church, we find a profound respect for the Sacred Heart of our Lord as a font of His love which gave birth to the Church and continues to nourish its members.
The devotion continued to grow during the Middle Ages and in 1353 Pope Innocent VI instituted a Mass honoring the mystery of the Sacred Heart. During the age of the Protestant movement, devotion to the Sacred Heart was practiced in hope of restoring peace to a world shattered by political and religious persecution. Shortly thereafter, the devotion escalated due to the fervor surrounding the apparitions of our Lord to St. Margaret Mary Alacoque (1647-90). On Dec. 27, 1673, our Lord revealed, "My Divine Heart is so passionately inflamed with love... that, not being able any longer to contain within Itself the flames of Its ardent charity, it must let them spread abroad through your means, and manifest Itself to man, that they may be enriched with Its precious treasures which I unfold to you, and which contain the sanctifying and salutary graces that are necessary to hold them back from the abyss of ruin." The four apparitions provided the catalyst for the promotion of the devotion to the Sacred Heart: a feast day in honor of the Sacred Heart and the offering of our Lord's saving grace and friendship if the individual attended Mass and received Holy Communion on nine consecutive first Fridays of the month. In 1899, Pope Leo XIII consecrated the world to the Sacred Heart of Jesus. Since then, his successors have exhorted the faithful to turn to the Sacred Heart and make acts of personal consecration. They have also begged the faithful to offer prayers and penances to the Sacred Heart in reparation for the many sins of the world.