61 We are aware of the difficulties of our times, and of the dire straits to which the ancient dioceses of Europe and America are reduced. But if we reason correctly, we can easily see that the poverty of some dioceses will appear to be abundant by comparison with the extreme need that prevails in others. Such a comparison, however, is without any point, since it is of less importance to balance statistics than to urge all the faithful, as We have done on another auspicious occasion, "to range themselves voluntarily beneath the Christian standards of abstinence and self-denial, beyond the requirements of the moral law, in accordance with each one's ability, as the grace of God inspires them and as their station in life permits." We also added, "let people devote to charity what they withdraw from vanities, and mercifully meet the needs of the Church and of her poor."
Source: Fidei Donum (Vatican.va)