Pope Pius XII's Meditations on the Large Family

"[A]s long as there is no sincere determination to let the Creator carry on His work as He chooses, then human selfishness will always find new sophistries and excuses to still the voice of conscience (to the extent it can), and to carry on abuses.
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[O]verpopulation is not a valid reason for spreading illicit birth control practices. It is simply a pretext used by those who would justify avarice and selfishness — by those nations, for instance, who fear that the expansion of others will pose a danger to their own political position and cause a lowering of the general standard of living, or by individuals, especially those who are better off, who prefer the greatest possible enjoyment of earthly goods to the praise and merit of bringing new lives into existence. The final result is that they break the fixed and certain laws of the Creator under the pretext of correcting supposed errors on the part of His Providence.
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As for you, parents and children of large families, keep on giving a serene and firm testimony of your trust in divine Providence, and be assured that He will not fail to repay you with the testimony of His daily help and, whenever necessary, with those extraordinary helps that many of you have been happy to experience already.
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The joy that comes from the plentiful blessings of God breaks out in a thousand different ways and there is no fear that it will end. The brows of these fathers and mothers may be burdened with cares, but there is never a trace of that inner shadow that betrays anxiety of conscience or fear of an irreparable return to loneliness, Their youth never seems to fade away, as long as the sweet fragrance of a crib remains in the home, as long as the walls of the house echo to the silvery voices of children and grandchildren.

Their heavy labors multiplied many times over, their redoubled sacrifices and their renunciation of costly amusements are generously rewarded even here below by the inexhaustible treasury of affection and tender hopes that dwell in their hearts without ever tiring them or bothering them.
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With good reason, it has often been pointed out that large families have been in the forefront as the cradles of saints. We might cite, among others, the family of St. Louis, the King of France, made up of ten children, that of St. Catherine of Siena who came from a family of twenty-five, St. Robert Bellarmine from a family of twelve, and St. Pius X from a family of ten.

Every vocation is a secret of Providence; but these cases prove that a large number of children does not prevent parents from giving them an outstanding and perfect upbringing; and they show that the number does not work out to the disadvantage of their quality, with regard to either physical or spiritual values."

--Pope Pius XII, The Large Family.