59 But "unless the Lord keep the city, he watcheth in vain that keepeth it." And so, as a final and most efficacious remedy, We recommend, Venerable Brethren, that in your dioceses you use the most practical means to foster and intensify the spirit of prayer joined with Christian penance. When the Apostles asked the Savior why they had been unable to drive the evil spirit from a demoniac, Our Lord answered: "This kind is not cast out but by prayer and fasting." So, too, the evil which today torments humanity can be conquered only by a world-wide crusade of prayer and penance. We ask especially the Contemplative Orders, men and women, to redouble their prayers and sacrifices to obtain from heaven efficacious aid for the Church in the present struggle. Let them implore also the powerful intercession of the Immaculate Virgin who, having crushed the head of the serpent of old, remains the sure protectress and invincible "Help of Christians."
Source: Divini Redemptoris (Vatican.va)