For many reasons. Marriage is a Sacrament, and those who desire to receive that Sacrament should be duly and validly baptized Christians. The Church, however, has no certainty that any non-Catholic has ever been validly baptized at all. Again, it is a sacrilege to receive a Sacrament whilst one is in a state of grave sin. The Catholic party prepares by a good confession, whilst the non-Catholic more often than not gives no thought whatever to the matter. Then, too, any children of the marriage have the bad example of one of the parents who never fulfills Catholic duties, even if the poor children be brought up as Catholics at all. The Catholic party is constantly subject to discouragement in the practice of his or her religion, and is even exposed to the danger of a complete loss of faith and of salvation in the end. Nor are mixed marriages, as a rule, in the interests of the parties themselves from the point of view of mutual happiness. Marriage is difficult enough in any case when the first glow of love begins to settle down to the realities of life. It is vastly more difficult when the Protestant does not understand Catholic ways, has no sympathy at all with the Catholic party on the most vital of all matters—religion, and even resents the claims of the Catholic Church. Mutual misunderstandings result, and the Protestant, not making the Catholic as happy as he expected, does not make himself as happy as he dreamed.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer CartyBoost your faith with the help of the Catholic book we suggest below. It is a helpful resource that answers a lot of questions and can be shared with family and friends.
The Case for Catholicism - Answers to Classic and Contemporary Protestant Objections
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