Pope Pius XI believed the devotion was the "sum of all religion" and provided a key to the virtuous life.
Devotion to the Sacred Heart has its roots in the Bible and has developed over the centuries. It was never a popular devotion until the the private revelations given to St. Margaret Mary Alacoque in the 17th century.
After that it became very popular among the faithful and the Church began to speak directly about this devotion, instituting a universal feast honoring the Sacred Heart.
When reflecting on this devotion in his encyclical, Miserentissimus Redemptor, Pope Pius XI wrote that it was a potent summary of the spiritual life.
For is not the sum of all religion and therefore the pattern of more perfect life, contained in that most auspicious sign and in the form of piety that follows from it inasmuch as it more readily leads the minds of men to an intimate knowledge of Christ Our Lord, and more efficaciously moves their hearts to love Him more vehemently and to imitate Him more closely? It is no wonder, therefore, that Our Predecessors have constantly defended this most approved form of devotion from the censures of calumniators, and have extolled it with high praise and promoted it very zealously, as the needs of time and circumstance demanded.
Pius XI explains how the devotion leads the faithful to an “intimate knowledge” and “love” of Jesus Christ.
The devotion highlights how “the creature’s love should be given in return for the love of the Creator.“
The Catechism of the Catholic Church affirms this central duty of all people, responding to the love of our Creator.
The dignity of man rests above all on the fact that he is called to communion with God. This invitation to converse with God is addressed to man as soon as he comes into being. For if man exists it is because God has created him through love, and through love continues to hold him in existence. He cannot live fully according to truth unless he freely acknowledges that love and entrusts himself to his creator.CCC 27
While not everyone is drawn closer to God in the Sacred Heart devotion, it remains a beautiful expression of a fundamental truth.
God loves us so much that he offers his love to us and invites us to love him in return.