4 With these and many other sweet memories in Our mind, and aware of the grave duties imposed upon the Supreme Shepherd of the flock of God, We would like, Venerable Brethren—seizing an occasion offered by that memorable Apostolic Letter , Maximum illud , (5) with which, forty years ago, Our predecessor Benedict XV furthered the cause of the Catholic missions by establishing new rules and enkindling the faithful with new zeal—We would like, We repeat, to speak to you with a fatherly heart, by means of this letter, on the necessity and hopes of extending God's kingdom to the many parts of the world where missionaries labor zealously, sparing no effort in order that new branches of the Church may grow and produce wholesome fruits.
Source: Princeps Pastorum (Vatican.va)