No one said that. However, you can’t force a 15 year old child to believe anything.
That’s why you don’t wait to begin their education until they’re 15, and why you don’t send them confusing messages when they are little. (You go to Hell if you don’t go to Church, but Daddy doesn’t go to Church - is Daddy going to Hell? Don’t you dare speak about your father like that! But, is he, or not? How come I can’t stay home with Daddy? If Daddy’s not going to Hell for not going to Church, then why would I go to Hell for not going to Church?)
By the time they are 15, it should just be an unquestioned assumption that “all normal people go to Church,” (because nobody they know at home has ever
not gone to Church) and that “Of course Jesus speaks to us through the Church; that’s why we have a Pope” and “It’s bad if people aren’t kind to each other.” You don’t wait until that time to start telling them about the Pope, and you don’t put them in a situation where someone they love and respect is going to disagree with basic fundamental ideas like that.
Have you tried? Your goal is to instill a sense of right and wrong, and teach them about God and the Church at a young age.
While your spouse is teaching them about a different church, or else modeling that church isn’t really all that important.
You can force them to go to Mass on Sundays, you can force them to go through the Sacraments, but what they believe after around 12 can’t be forced.
What they believe at age 12 will be whatever they were shown by both parents (not merely told) between the ages of 2 and 5. As an anecdotal experience, in my case, both of my parents were church-goers when I was that age. By the time my brother was about 3 or 4 my father had stopped attending services. I continued to attend Church throughout my teen years because it had been rooted deep into my psyche that “you just go to Church and that’s that” - my brother didn’t - he had been shown that, “Well, it’s not required, if you think you are too busy for it”. Those 3-4 years when the rational mind is developing are absolutely crucial, I think.
If both parents have similar enough faiths that children are taught consistently and uniformly through age 12, then there’s not a problem.
Which is why both partners should be Catholics. There are enough differences between Catholics that the child will get all the “choices” he needs in life. Besides, it doesn’t matter how similar your partner’s church is to the Catholic Church; your child will certainly notice that he or she is not going to Mass with the rest of the family.
After that, you can’t force any belief, but rather try to explain the situation and hope that they were taught properly and accept truth.
You have two parents teaching the child. Each with equal authority. How is the child to know which one is teaching “the truth”? And how is it teaching properly, to present two different points of view as if they were both equally valid? (Or worse, to tell your child that her other parent’s faith isn’t valid.)
One thing that’s also missing in the discussion between Catholics and non-Catholics marrying is that there’s a very high conversation rate among non-Catholics (I don’t have any statistics, but that’s anecdotal).
There is, but there is also an extremely high recidivism rate among adults who convert for marriage - usually, the reason is so that their spouse can be married in the Church and so that their children can go to Catholic schools, and once that goal has been accomplished, they don’t really continue in the practice of the faith.
Those who convert because they have become convinced of the claims of the Catholic Church are a totally different story, of course.