10 This being so, Venerable Brethren, We consider it Our duty to stimulate the pious efforts and charity of Christians, so that they may strive, whether by prayer or by donations, to help the sacred work of missions and to show favour to the propagation of the faith. The good which it is proposed to secure, and the fruits to be gathered, prove the importance of this holy enterprise. For this work tends directly to the glory of the Divine name and to the spread of the Kingdom of Christ upon earth. But it is incredibly beneficial to those who are called out of the filth of vice and the shadow of death; and who, being made partakers of eternal life, are also brought out of barbarism and a state of savage manners into the fulness of civilised life. Moreover, it is highly useful and advantageous to those who take any part in it, since it procures them spiritual riches, supplies them with an occasion of merit, and renders, as it were, God himself their debtor.
Source: Sancta Dei Civitas (Vatican.va)