The Rules Kate Middleton Has To Follow Prove Princess Life Is Not Easy!

She is only allowed to be a Protestant Anglican. 

Kate, like the rest of the Royal Family, is unable to practice any religion other than Protestant Anglican. It would be awkward if someone in the Royal family professed a different religion because the king of Great Britain is also the supreme leader of the Protestant Anglican Church. 

As part of her preparations for her marriage to Prince William, the Duchess underwent ecclesiastical confirmation in the Anglican Church in 2011.