A historic oddity:

""The British Monarch can now marry a Roman Catholic, but he or she cannot become one.In  recent years, someone in line for the throne could marry anyone, but not a Roman Catholic, and still retain his or her place in the line of succession. The person could marry a Buddhist, a Muslim, a Jew, a Zoroastrian and retain that place, but though you COULD marry a Catholic, doing so would result in loss of one's place in the line. A number of people in the extended royal family have lost their places in the succession as a result of marrying Catholics or converting to Catholicism. 

The law has changed. In future, heirs will be able to retain their places in the line even if they marry Catholics. However, since the monarch, and head of the British Commonwealth, must be "in communion" with the Church of England, an heir cannot convert to another faith without giving up his or her place in the line, and the children will have to be raised C of E, lest they be ineligible for the throne. ""