In 1931, Jesus appeared to a young Polish nun, Saint Faustina Kowalska, with rays radiating from His heart. Jesus said, "Paint an image according to the pattern you see with the signature: Jesus I Trust in You."
Thus He began a devotion the Church has always taught through Scripture and Sacred Tradition: that God is merciful and forgiving to all and that we, too, must receive His mercy and bring it to others.