Traditionally, Muslim men are allowed to marry women who are from the “People of the Book,” generally defined as Christians and Jews. In this case, a Muslim husband must guarantee the right of his Christian or Jewish wife to worship God according to her religious beliefs.
The reverse, i.e., a Muslim woman marrying a man outside her religion, has traditionally not been allowed on the grounds that her husband might not guarantee her the right to practice her religion, since he may not to have the same obligation to respect her religion that a Muslim has towards his Christian or Jewish wife. Therefore, for the protection of her freedom of religion, a Muslim woman has traditionally been required to marry a man who will give her the right to practice her faith—that is, a Muslim.
Today, especially in areas where Muslims live as minorities, there is growing diversity in both theory and practice on this issue.