11 Above all, one must be firmly convinced that the instrument is of little value if it is not adapted to the work at hand. In regard to the things We mentioned above, Catholic Action, inasmuch as it proposes to restore all things in Christ, constitutes a real apostolate for the honor and glory of Christ Himself. To carry it out right one must have divine grace, and the apostle receives it only if he is united to Christ. Only when he has formed Jesus Christ in himself shall he more easily be able to restore Him to the family and society. Therefore, all who are called upon to direct or dedicate themselves to the Catholic cause, must be sound Catholics, firm in faith, solidly instructed in religious matters, truly submissive to the Church and especially to this supreme Apostolic See and the Vicar of Jesus Christ. They must be men of real piety, of manly virtue, and of a life so chaste and fearless that they will be a guiding example to all others. If they are not so formed it will be difficult to arouse others to do good and practically impossible to act with a good intention. The strength needed to persevere in continually bearing the weariness of every true apostolate will fail. The calumnies of enemies, the coldness and frightfully little cooperation of even good men, sometimes even the jealousy of friends and fellow workers (excusable, undoubtedly, on account of the weakness of human nature, but also harmful and a cause of discord, offense and quarrels) - all these will weaken the apostle who lacks divine grace. Only virtue, patient and firm and at the same time mild and tender, can remove or diminish these difficulties in such a way that the works undertaken by Catholic forces will not be compromised. The will of God, Saint Peter wrote the early Christians, is that by your good works you silence the foolish. "For such is the will of God, that by doing good you should put to silence the ignorance of foolish men."
Source: Il Fermo Proposito (Vatican.va)