The Rules Kate Middleton Has To Follow Prove Princess Life Is Not Easy
She Can Only Be Protestant Anglican

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Like other members of the Royal Family, Kate cannot belong to any religion other than the Protestant Anglican. The monarch of Great Britain is also the supreme head of the Protestant Anglican Church, so it would be pretty awkward if someone in the Royal family practiced a different religion.
In 2011, the Duchess went through a religious confirmation ceremony at the Anglican Church as part of preparations for her marriage to Prince William.