If the coming of the Holy Spirit upon the Apostles at Pentecost marks the hour of the birth of the Church, then His descent upon Mary in Nazareth marks the beginning of the work of salvation (cf. Lk 1:35).
The Church teaches us that Mary was preserved from original sin in view of the Saviour who would be born of her. This is the dogma of the Immaculate Conception: that the Virgin Mary, by a special grace of God, preserved the state of paradisiacal innocence.