On December 17th, 1927, she went before the tabernacle to ask Jesus how she should comply with what had been asked of her, that is, to say if the origin of the devotion to the Immaculate Heart of Mary was included in the Secret that the most holy Virgin had confided to her.

Jesus made her hear very distinctly these words:

— My daughter, write what they ask of you. Write also all that the most holy Virgin revealed to you in the Apparition, in which she spoke of this devotion. As for the remainder of the Secret, continue to keep silence.

What was confided on this subject in 1917, is as follows:

She asked for them to be taken to Heaven, and the most holy Virgin answered:

— Yes. I will take Jacinta and Francisco soon. But you are to stay here some time longer. Jesus wishes to make use of you to make me known and loved. He wants to establish in the world devotion to my Immaculate Heart. I promise salvation to those who embrace it, and these souls will be loved by God, like flowers placed by me to adorn His throne.

— Am I to stay here all alone? — she asked, sadly.

— No, daughter. I shall never forsake you. My Immaculate Heart will be your refuge and the way that will lead you to God.

On December 10, 1925, the Most Holy Virgin appeared to her, and by Her side, elevated on a luminous cloud, was the Child Jesus. The Most Holy Virgin rested Her hand on her shoulder, and as She did so, She showed her a heart encircled by thorns, which She was holding in Her other hand.

At the same time, the Child said:

— Have compassion on the Heart of your Most Holy Mother, covered with thorns, with which ungrateful men pierce it at every moment, and there is no one to make an act of reparation to remove them.

Then the Most Holy Virgin said:

— Look, My daughter, at My Heart, surrounded with thorns with which ungrateful men pierce Me every moment by their blasphemies and ingratitude. You at least try to console Me and say that I promise to assist at the hour of death, with the graces necessary for salvation, all those who, on the first Saturday of five consecutive months, shall confess, receive Holy Communion, recite five decades of the Rosary, and keep Me company for fifteen minutes while meditating on the fifteen mysteries of the Rosary, with the intention of making reparation to Me.

On February 15th, 1926, the Infant Jesus appeared to her again. He asked her if she had already spread the devotion to His most holy Mother. She told Him of the confessor’s difficulties, and said that Mother Superior was prepared to propagate it, but that the confessor had said that she, alone, could do nothing.

Jesus replied:

— It is true that your Superior alone can do nothing, but with my grace, she can do all.

She placed before Jesus the difficulty that some people had about confessing on Saturday, and asked that it might be valid to go to confession within eight days. Jesus answered:

— Yes, and it could be longer still, provided that, when they receive Me, they are in the state of grace and have the intention of making reparation to the Immaculate Heart of Mary.

She then asked:

— My Jesus, what about those who forget to make this intention?

Jesus replied:

— They can do so at their next confession, taking advantage of the first opportunity to go to confession.

Sister Lucia's Memoirs. [pdf]
Text of the great promise of the Heart of Mary