While many Fathers and Doctors of the Church considered Mary the greatest and holiest of the saints, they often had difficulty recognizing Mary as sinless—both at her conception and throughout her life. This Church teaching arose from the piety of the faithful.
Eventually, two Franciscans, William of Ware and Blessed John Duns Scotus, helped to develop the theology. They argued that Mary’s Immaculate Conception enhances Jesus’ redemptive work. Other members of the human race are cleansed from original sin after birth. In Mary, Jesus’ work was so powerful that it prevented original sin at the beginning.
This great privilege of Mary is the highlight of all that God has done in Jesus. Correctly understood, the incomparable holiness of Mary shows forth the incomparable goodness of God.