From Catholic Tradition:
DEVOTION TO THE SOULS IN PURGATORY AND THE DEVOTION TO THE SACRED HEART

Among the devotions which should form a crown of honor around the Sacred Heart, St. Margaret Mary gives a privileged place to devotion to the Souls in Purgatory. She herself had an extraordinary devotion to these Souls and called them her good suffering friends. The revelations which she received about the sufferings of these Souls, about our Divine Lord’s tender love for them and His eager desire for their deliverance, of the great efficacy of the devotion to the Sacred Heart for their early release, and the fact that St. Margaret Mary, the apostle of the public devotion to the Sacred Heart, combined these two devotions so intimately in her own person, indicate that there is such a close connection between the two devotions that the devotion to the Souls in Purgatory may be said to form a part of the devotion to the Sacred Heart. Her extraordinary compassion for these suffering Souls may be said to be a share of the compassion of the Divine Heart for them; she called the devotion to the Sacred Heart the sovereign remedy for their relief.

It is a remarkable fact that our Divine Lord demanded of the two pioneers of the public devotion to His Sacred Heart an act of consecration of themselves to It, and a total donation in favor of the Souls in Purgatory of all the satisfactory merits of their lives and of all the prayers, Masses and good works offered for them after their death, and that they were the first to make this heroic offering.

St. Margaret Mary was in constant communication with the Souls in Purgatory; our Divine Lord allowed many privileged Souls among them to visit her, to tell her of their great sufferings and what it was that caused them, and to appear to her in glory after their deliverance. Towards the end of her life this became known outside the convent and people came to inquire from her about their deceased friends. While she usually declined to give any reply at the time of the inquiry, later on she sometimes declared that such a person was now in Heaven and exhorted the relatives of others to continue their prayers.

MARVELOUS EFFICACY OF THE DEVOTION TO THE SACRED HEART TO SUCCOR THE SOULS IN PURGATORY

“Would that people knew;’ writes St. Margaret Mary, “with what eagerness the poor Souls in Purgatory ask for this new remedy which is so powerful to relieve their sufferings. They call the Devotion to the Sacred Heart the ‘Sovereign remedy,’ and ask particularly for Masses in honor of the Sacred Heart of Jesus.”

ST. MARGARET MARY RECOMMENDS NOVENAS IN HONOR OF THE SACRED HEART FOR THE SOULS IN PURGATORY

Writing to Mother de Saumaise she says: “There is a Soul in Purgatory that I cannot comfort as much as I desire. She told me to apply to you and ask you to have three Masses said for her, and, in addition, to offer your Rosary and all the practices of our holy rule for nine days, and a general
Communion. She said that these things would greatly alleviate her pains and that she would not be ungrateful.”

ST. MARGARET MARY WAS ACCUSTOMED TO OFFER HER PRAYERS TO OUR BLESSED LADY THROUGH THE SOULS IN PURGATORY, AND IN A LETTER TO MOTHER GREFIÉ SHE EXPLAINS THE REASON:

“It is not through any want of confidence in Our Blessed Lady that we ask the help of the Holy Souls in Purgatory; it is in order that they may join their supplications to ours and offer them up to this Good Mother to obtain for us the continuation of her maternal assistance. By doing so we confess that, after God, it is from the Blessed Virgin that all good comes to us.”
Of all the practices for the relief of the Holy Souls, the most efficacious is that which is called “the Heroic Act of Charity,” which, as already stated, both St. Margaret Mary and Blessed Claude de la Colombiere made at our Divine Lord’s request. It consists in offering up in favor of the Souls in Purgatory all our satisfactory merits and all the indulgences we can gain during our lives, and all the prayers, Masses and good works offered for us after our death, thus reserving nothing for ourselves and trusting completely in the mercy of the Sacred Heart of Jesus. St. Margaret Mary makes reference to it in several of her letters.

PRAYER FOR THE HOLY SOULS

O Divine Heart of Jesus, grant I pray Thee, eternal rest to the Souls in Purgatory, the final grace to those who are about to die this day, true repentance to sinners, the light of faith to pagans, and Thy blessing to me and to all who are dear to me. To Thee, therefore, O Most Merciful Heart of Jesus, I commend all these souls, and in their behalf I offer unto Thee all Thy merits in union with the merits of Thy Most Blessed Mother and of all the Angels and Saints, together with all the Masses, Communions, prayers and good works which are this day being offered throughout Christendom.
O Most Holy Heart of Jesus, shower Thy blessings in abundant measure upon the Thy Holy Church, upon the Supreme Pontiff and upon all the clergy; to the just grant perseverance; convert sinners; enlighten unbelievers; bless our relations, friends and benefactors; assist the dying; deliver the Holy Souls in Purgatory; and extend over all hearts the sweet empire of Thy love. Amen.