With the proper permissions from one’s local bishop, a Catholic can marry a non-Catholic Christian or a non-Christian. That said, the Church does not encourage interfaith marriages because they are extremely difficult. Some couples are heroically able to make them work, but such cases are ordinarily quite rare. What usually happens is that the faith of both partners suffers and that the children usually end up without any solid faith.
It is an especially bad idea to enter a relationship with the idea of converting someone else. Even if you were a Catholic yourself, this would be a bad idea because no one should convert for the sake of marriage. The only reason to enter the Catholic Church is because one is convinced in conscience that it is the Church founded by Christ and that one is duty-bound as a follower of Christ to join his Church. While a relationship with a Catholic might be a reason to investigate Catholicism, it is *not *sufficient reason to convert.
It is particularly problematic that you, as an inquirer into the Catholic faith, are seeking to convert someone from Seventh-Day Adventism, which is a strongly anti-Catholic Christian denomination. Because your friend is already a believer in her religion and you are only an inquirer into Catholicism, she is more likely to be able to convert you to Seventh-Day Adventism than you are to convert her to Catholicism. Right now, your priority as a follower of Jesus Christ should be to focus your attention on investigating Catholicism and entering the Catholic Church if you find that you are convinced of the truth of Catholicism. In other words, your attention should be on your relationship with Jesus Christ. There will be more than enough time later to find out whether this person is the right person to be your wife.
Marriage is a vocation, as is holy orders, and is intended for the sanctification of the spouses and for raising children for God. It is best for Catholics to choose fellow Catholics for spouses because another committed, believing Catholic will best be able to help a fellow Catholic fulfill his vocation to marriage.
Recommended reading:
How to Become a Catholic
Seventh-Day Adventism
Seventh-Day Adventism: A Catholic Perspective by William J. Cork
