My sister (raised WELS Lutheran) married a Catholic man over ten years ago. She has continued as a WELS member, and he as a Catholic. They have gone back and forth attending Lutheran and Catholic churches over the years, but haven't officially joined one (they have moved several times). They just recently had their third child - the first was baptized Lutheran, the second was baptized Catholic, and the third is going to be baptized Catholic. Is it better for one of them to convert and raise their family under one denomination? Or, do they remain separate?
Your questions about faith are very important, and your concern for family members is commendable. I can address your questions, and you can then determine what your concern for family leads you to do.
For starters, we want to understand what happens when churches baptize. A Trinitarian baptism brings a person into the Holy Christian Church. That says much more than someone was “baptized Catholic” or “baptized Lutheran.” Baptism brings a person into the Christian Church.
But churches do more than baptize; they teach. And at some point, baptized individuals are given the opportunity to profess the teachings of the church that has taught them. Confirmation, for example, is a time when individuals profess that the teachings they have learned from their instruction in the Lutheran Church are the teachings of the Bible and are correct.
Obviously, the teachings of churches differ. In your question you are referencing the teachings of the Roman Catholic Church and the Lutheran Church. A key is understanding that the differences between the churches are teachings—doctrines—and not “philosophies.” A major difference of doctrine is the way of salvation. Is salvation God’s gift, offered freely through his Son Jesus Christ, and received in faith? Or, is salvation a combination of faith and works? Those questions paint a picture of the huge contrast between Lutheran teaching and Roman Catholic teaching.
If a husband and wife hold to these different beliefs, a great unity in marriage is missing. “Mixed” marriages like that might “work well,” as you indicated, but how so? Often, they “work well” in that husband and wife are loving toward each, they raise obedient, respectful children and outwardly they have a happy home life.
But marriages like that do not “work well” when it comes to spouses trying to worship and commune together, and providing unified spiritual direction to their children. What “works” much better is when husband and wife have a common faith and a common membership.
I do not know what your sister and her husband have done to examine the teachings of their churches. They would benefit greatly from a study and discussion of what each church teaches about the Bible. There are several books from our publishing house that could assist them in that study. Keep in mind also that our churches regularly offer no-obligation Bible Information Classes, where people can receive an overview of the Bible’s teachings and answers to their questions.