From: 'Seminarian's Guide'
The mosaic altarpiece was blessed at St. Margaret Mary Alacoque's canonization in 1923. She was the saint responsible for the spread of this popular devotion to Our Lord's Sacred Heart.

When St. Margaret Mary approached her spiritual director and told him that she had been visited by Our Lord, he was uncertain about her sincerity. He told her to ask Jesus, next time He appeared, to recount some of her past sins. During her next visit, somewhat puzzled, she reported the result: Our Lord had told her that He did not remember any of her sins! Then her spiritual director knew that she was telling the truth, since God's forgiveness is utter and unconditional.

From: 'St. Peter's Basilica - A Virtual Tour' by Our Sunday Visitor
Facing it is the Sacred Heart Altar, with the mosaic altarpiece illustrating the "Apparition of the Heart of Jesus to St. Mary Margaret," executed in the 1920s. It is without a doubt one of the less successful works in the Basilica, both because of the banal conventionality of its inspiration and the rash mediocrity of its execution.

It replaced the "Fall of Simon the Magician," painted on slate by Francesco Vanni, and commissioned by Cardinal Baronio, who saw the episode as a victory of the Papacy over Protestantism. This painting was briefly substituted by another by P. Batoni on the same subject, now in Santa Maria degli Angeli. Vanni's painting was restored and put back in place until 1921, but it soon deteriorated once again. Then C. Muccioli was commissioned to create the actual painting, later put into mosaic by several artists.