In biblical language, the “heart”‌ indicates the centre of human life, the point where reason, will, temperament and sensitivity converge, where the person finds his unity and his interior orientation. According to Matthew 5:8, the “immaculate heart”‌ is a heart which, with God's grace, has come to perfect interior unity and therefore “sees God”‌. To be “devoted”‌ to the Immaculate Heart of Mary means therefore to embrace this attitude of heart, which makes the fiat – “your will be done”‌ - the defining centre of one's whole life.

(…) “my Immaculate Heart will triumph”‌. What does this mean? The Heart open to God, purified by contemplation of God, is stronger than guns and weapons of every kind. The fiat of Mary, the word of her heart, has changed the history of the world, because it brought the Saviour into the world - because, thanks to her Yes, God could become man in our world and remains so for all time. The Evil One has power in this world, as we see and experience continually; he has power because our freedom continually lets itself be led away from God. But since God himself took a human heart and has thus steered human freedom towards what is good, the freedom to choose evil no longer has the last word. From that time forth, the word that prevails is this: “In the world you will have tribulation, but take heart; I have overcome the world” (Jn 16:33). The message of Fatima invites us to trust in this promise.

Joseph Card. Ratzinger