3 In return for so divine a gift as this, after the due submission of his mind, what can a man do that will be more acceptable to God than to carry far and wide among his fellowmen the torch of truth that Christ brought to Us? By their zeal in promoting the sacred missionary efforts of the Church, a zeal that generously feeds the fire of Christian charity, men, ever mindful of the gift of faith, may in some way make a return to Almighty God; by so doing and imparting to others according to their ability the gift of the faith that is theirs, they are visibly manifesting their gratitude to the Heavenly Father.
Source: Fidei Donum (Vatican.va)