20 Every true and lasting reform has ultimately sprung from the sanctity of men who were driven by the love of God and of men. Generous, ready to stand to attention to any call from God, yet confident in themselves because confident in their vocation, they grew to the size of beacons and reformers. On the other hand, any reformatory zeal, which instead of springing from personal purity, flashes out of passion, has produced unrest instead of light, destruction instead of construction, and more than once set up evils worse than those it was out to remedy. No doubt "the Spirit breatheth where he will" ( John iii. 8): "of stones He is able to raise men to prepare the way to his designs" ( Matt . iii. 9). He chooses the instruments of His will according to His own plans, not those of men. But the Founder of the Church, who breathed her into existence at Pentecost, cannot disown the foundations as He laid them. Whoever is moved by the spirit of God, spontaneously adopts both outwardly and inwardly, the true attitude toward the Church, this sacred fruit from the tree of the cross, this gift from the Spirit of God, bestowed on Pentecost day to an erratic world.
Source: Mit Brennender Sorge (Vatican.va)