This topic of the research was basically not undertaken yet in the theological literature. St. Faustina offers a definition of sanctity and shows its development till the point of achieving a mystical union. It is mystical sanctity, therefore, which ultimate norm is the unity with Jesus. In this line, Faustina trusts that God’s Mercy will make her holy. In this context we can understand that it was merciful holiness expressed by receiving mercy and rejecting sin. Christ is introducing her in the Mystery of His Passion. Pasha of the sanctity which is achieved by the Christian through the bond with Jesus, causes diving in His death and vocation to life in His resurrection. Therefore, it is a paschal sanctity. According to that, the paschal Mystery makes up the center of the mystical life of St. Faustina. This results that she puts heavy accent on this mystical union with Christ’s suffering, with His death on the Cross. She discovers her calling to sanctity under the influence of Pasha as a call to become a holistic victim offered to God. This work of sanctity we can call the Mystic of the merciful sanctity. And Faustina writes about this too, and not only about the Mercy.