The mysteries of Jesus are not yet completely perfected and fulfilled. They are complete, indeed, in the person of Jesus, but not in us, who are his members, nor in the Church, which is his mystical body. The Son of God wills to give us a share in his mysteries and somehow to extend them to us. He wills to continue them in us and in his universal Church. This is brought about first through the graces he has resolved to impart to us and then through the works he wishes to accomplish in us through these mysteries. This is his plan for fulfilling his mysteries in us.
For this reason Saint Paul says that Christ is being brought to fulfillment in his Church and that all of us contribute to this fulfillment, and thus he achieves the fullness of life, that is, the mystical stature that he has in his mystical body, which will reach completion only on judgment day. In another place Paul says: I complete in my own flesh what is lacking in the sufferings of Christ.
This is the plan by which the Son of God completes and fulfills in us all the various stages and mysteries. He desires us to perfect the mystery of his incarnation and birth by forming himself in us and being reborn in our souls through the blessed sacraments of baptism and the Eucharist. He fulfills his hidden life in us, hidden with him in God.
--St. John Eudes