“Let us realize all the richness hidden in the words “the sacred heart of Jesus.” When we speak of a person’s heart, we refer not just to his sentiments, but to the whole person in his loving dealings with others. In order to help us understand divine things, Scripture uses the expression “heart” in its full human meaning, as the summary and source, expression and ultimate basis, of one’s thoughts, words and actions. A man is worth what his heart is worth…
To the heart belongs joy: “let my heart rejoice in your saving help”; repentance: “my heart is like wax, it is melted within my breast”; praise of God: “my heart overflows with a goodly theme”; the decision to listen to the Lord: “my heart is ready, Lord”; loving vigilance: “I slept, but my heart was awake”; and also doubt and fear: “let not your hearts be troubled, believe in me.”
The heart not only feels, it knows and understands. God’s law is received in the heart and remains written there. Scripture also adds: “Out of the abundance of the heart the mouth speaks.” Our Lord reproaches the scribes: “Why do you think evil in your hearts?” And, summing up all the sins man might commit, he says: “Out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander and blasphemy.”
When holy Scripture refers to the heart, it does not refer to some fleeting sentiment of joy or tears. By heart it means the personality which directs its whole being, soul and body, to what it considers its good, as Jesus himself indicated: “For where your treasure is, there will your heart be also.”
So when we talk about the heart of Jesus, we stress the certainty of God’s love and the truth of his commitment to us. When we recommend devotion to the sacred heart, we are recommending that we should give our whole self to Jesus, to the whole Jesus — our soul, our feelings and thoughts, our words and actions, our joys.
That is what true devotion to the heart of Jesus means. It is knowing God and ourselves. It is looking at Jesus and turning to him, letting him encourage and teach and guide us. The greatest superficiality that can beset this devotion would be a lack of humanity, a failure to understand the reality of an incarnate God. ”

– St. Josemaria Escriva