87 No less far from the truth is the dangerous error of those who endeavor to deduce from the mysterious union of us all with Christ a certain unhealthy quietism . They would attribute the whole spiritual life of Christians and their progress in virtue exclusively to the action of the Divine Spirit, setting aside and neglecting the collaboration which is due from us. No one, of course, can deny that the Holy spirit of Jesus Christ is the one source of whatever supernatural powers enters into the Church and its members. For "The Lord will give grace and glory" as the Psalmist says. But that men should persevere constantly in their good works, that they should advance eagerly in grace and virtue, that they should strive earnestly to reach the heights of Christian perfection and at the same time to the best of their power should stimulate others to attain the same goal, - all this the heavenly Spirit does not will to effect unless they contribute their daily share of zealous activity. "For divine favors are conferred not on those who sleep, but on those who watch," as St. Ambrose says. For if in our mortal body the members are strengthened and grow through continued exercise, much more truly can this be said of the social Body of Jesus Christ in which each individual member retains his own personal freedom, responsibility, and principles of conduct. For that reason he who said: "I live, now not I, but Christ liveth in me" did not at the same time hesitate to assert: "His (God's) grace in me has not been void, but I have labored more abundantly than all they: yet not I, but the grace of God with me." It is perfectly clear, therefore, that in these false doctrines the mystery which we are considering is not directed to the spiritual advancement of the faithful but is turned to their deplorable ruin.
Source: Mystici Corporis Christi (Vatican.va)