In the Diary of St. Faustina Kowalska, the Polish nun described an image revealed to her of the risen Lord, with two rays shining from his heart: red representing blood and white symbolizing water, with the words “Jesus, I trust in you” underneath. St. Faustina wrote in her Diary that Jesus told her, “I promise that the soul that will venerate this image will not perish.”
When St. Faustina was canonized in 2000 by fellow countryman Pope John Paul II, he proclaimed that the Second Sunday of Easter would henceforth be known as Divine Mercy Sunday, thereby widely promoting the devotional practices associated with Faustina’s visions, already popular in many communities.
The above story from the March 25, 2002, B.C. Catholic describes the growing devotion to the Divine Mercy just two years after it was added to the Church calendar.