Catholic devotion to the Sacred Heart of Jesus developed over the centuries. One of its earliest sources from the Fathers of the Church was contemplation of the piercing of Jesus's heart on the cross, the wound from which blood and water flowed. Those signs of sacred water and saving blood, of course, have long been seen as representing Baptism and Holy Communion. This EWTN transcription of an article from 1913 Catholic Encyclopedia provides some background on the development of this devotion.
The modern celebration of the Solemnity of the Sacred Heart of Jesus and other devotions like First Fridays and Eucharistic Adoration on the Thursday before them, stem from the visions of St. Margaret Mary Alacoque in the 17th century in France. Pope Pius IX established the Feast of the Sacred Heart on the Friday after the Feast of Corpus Christi in 1856. Pope Leo XIII consecrated the entire human race to the Sacred Heart of Jesus on June 11, 1899.
As an Anglican, Newman knew little, probably, of this devotion, but as a Catholic, he certainly celebrated the Feast as Pope Pius IX had established just eleven years after his conversion. Since as a Cardinal he chose the motto of Cor ad Cor Loquitor, it makes sense that this devotion would mean something to him. (Note the two hearts above the one on his Cardinalate shield--one of them represents the Immaculate Heart Mary, the memorial following the Solemnity of the Sacred Heart.)
There are two readily available documents showing Newman's devotion to the Sacred Heart of Jesus and his celebration of the feast. The first is from his Meditations and Devotions. These meditations and devotions were published after his death, but according to his secretary at the Birmingham Oratory, Newman had intended to prepare a year-long devotional for the use of the boys at the Oratory School. You'll notice how personal and direct this prayer of adoration is, based on the doctrine of the Incarnation of Our Lord, the Second Person of the Trinity, as truly God and truly man.
1. O SACRED Heart of Jesus, I adore Thee in the oneness of the Personality of the Second Person of the Holy Trinity. Whatever belongs to the Person of Jesus, belongs therefore to God, and is to be worshipped with that one and the same worship which we pay to Jesus. He did not take on Him His human nature, as something distinct and separate from Himself, but as simply, absolutely, eternally His, so as to be included by us in the very thought of Him. I worship Thee, O Heart of Jesus, as being Jesus Himself, as being that Eternal Word in human nature which He took wholly and lives in wholly, and therefore in Thee. Thou art the Heart of the Most High made man. In worshipping Thee, I worship my Incarnate God, Emmanuel. I worship Thee, as bearing a part in that Passion which is my life, for Thou didst burst and break, through agony, in the garden of Gethsemani, and Thy precious contents trickled out, through the veins and pores of the skin, upon the earth. And again, Thou hadst been drained all but dry upon the Cross; and then, after death, Thou wast pierced by the lance, and gavest out the small remains of that inestimable treasure, which is our redemption.
2. My God, my Saviour, I adore Thy Sacred Heart, for that heart is the seat and source of all Thy {413} tenderest human affections for us sinners. It is the instrument and organ of Thy love. It did beat for us. It yearned over us. It ached for us, and for our salvation. It was on fire through zeal, that the glory of God might be manifested in and by us. It is the channel through which has come to us all Thy overflowing human affection, all Thy Divine Charity towards us. All Thy incomprehensible compassion for us, as God and Man, as our Creator and our Redeemer and Judge, has come to us, and comes, in one inseparably mingled stream, through that Sacred Heart. O most Sacred symbol and Sacrament of Love, divine and human, in its fulness, Thou didst save me by Thy divine strength, and Thy human affection, and then at length by that wonder-working blood, wherewith Thou didst overflow.
3. O most Sacred, most loving Heart of Jesus, Thou art concealed in the Holy Eucharist, and Thou beatest for us still. Now as then Thou savest, Desiderio desideravi—"With desire I have desired." I worship Thee then with all my best love and awe, with my fervent affection, with my most subdued, most resolved will. O my God, when Thou dost condescend to suffer me to receive Thee, to eat and drink Thee, and Thou for a while takest up Thy abode within me, O make my heart beat with Thy Heart. Purify it of all that is earthly, all that is proud and sensual, all that is hard and cruel, of all perversity, of all disorder, of all deadness. So fill it with Thee, that neither the events of the day nor the circumstances of the time may have power to ruffle it, but that in Thy love and Thy fear it may have peace.
Then, in his Sermon Notes, for Newman had started to give more feverino type of sermons, not writing them through to be read, but preparing numbered lists of what he wanted to say, he offers an explanation of the practice of this devotion on the Feast of the Sacred Heart on June 6, 1875.
His second point introduces the doctrinal basis of this devotion: "Our Lord is One. He is the one God. He took on Him a manhood, a body and soul; that body from Mary. Still, He was one, not two—one, as each of us is one."
Newman makes a comparison with the way that we love a certain attribute of someone we love because we love him or her and how we can love the Heart of Jesus because will love Him:
During Eucharistic Adoration at a Holy Hour, or Forty Hours Devotion, or private prayer before the Blessed Sacrament in a monstrance in an Adoration Chapel, we not idolize the Host as a separate object: we adore Jesus, the Son of God, the Second Person of the Trinity, present Body, Blood, Soul and Divinity in that Host or in that Tabernacle, veiled from sight.
Newman's last three points are:
8. What is the Heart the symbol of?—of His love, His affection for us, so that He suffered for us—the agony in the garden.
9. Moreover, of His love in the Holy Eucharist.
10. The Heart was the seat, first, of His love for us; secondly, of His many griefs and sorrows.
Trying to imagine what Father Newman was saying in developing these notes further is an interesting and humbling effort--I look forward to what Matt Swaim will draw from this exercise!--but we can see that Newman wanted to show his congregation how devotion to the Sacred Heart of Jesus helped them realize how much Jesus loved and loves them: He suffered and died for us; He comes to us in Holy Communion. Newman, for whom religious devotions were always based on the teachings of Jesus and His Church, made the connections between how we believe in Jesus and how we demonsrate our devotions to Him in prayer and worship of His Sacred Heart and in the Eucharist.
The Fathers of the Birmingham Oratory collected and published these Sermon Notes in 1913 with some comments about Father Newman and later Cardinal Newman's delivery of them in the Introduction:
Sacred Heart of Jesus, have mercy on us!
Saint John Henry Newman, pray for us!
Image credit (public domain): Sacred Heart of Jesus, Portuguese painting from the 19th century.