9 Nevertheless, it is evident that the apostolate thus understood does not come from a purely natural impulse to action, but is the fruit of a solid interior formation: it is the necessary expansion of an intense love of Jesus Christ and of souls redeemed by His Precious Blood, which is actuated by studying to imitate His life of prayer, of sacrifice, of inextinguishable zeal. This imitation of Christ will excite multiple forms of apostolate in every field, wherever souls are in danger or the rights of the Divine King compromised; it will extend to all the works of the apostolate, which in any manner enter into the divine mission of the Church, and consequently will penetrate not only the soul of each individual, but also into the sanctuary of the family, the school and even public life.
Source: Firmissimam Constantiam (Vatican.va)