The Challenge and Conquest YouCat number for October is about Mercy which is the message of St Faustina.
#314 How do we know that God is merciful? In many passages in Sacred Scripture God shows that he is merciful, especially in the parable of the merciful father who goes out to meet his prodigal son, accepts him unconditionally, and celebrates his return and their reconciliation with a joyful banquet. Jesus is sent “to the lost sheep of the house of Israel” and he knows that “Those who are well have no need of a physician, but those who are sick”