The devotion to Jesus as the Divine Mercy is based on the writings of Saint Faustina, a Polish nun who wrote a diary of about 600 pages recording the revelations she received about God's mercy.

Jesus looked at me and said, Souls perish in spite of My bitter Passion. I am giving them the last hope of salvation; that is, the Feast of My Mercy. If they will not adore My mercy, they will perish for all eternity. Secretary of My mercy, write, tell souls about this great mercy of Mine, because the awful day, the day of My justice, is near. (Diary - Divine Mercy in My Soul, 965)