The second trump was called La Papesse, the female pope, and it is the most historically loaded card in the deck. A female pope in a fifteenth-century Italian card game is not a neutral image. Two explanations circulate and neither is settled.
The first is the legend of Pope Joan, the woman who according to a widely repeated medieval story concealed her sex and reigned as pope. The story is not historically true, but it was believed and depicted, and a card-maker could reasonably have expected players to recognise it.
The second is more specific and more interesting. Sister Manfreda was a member of the Guglielmite sect in Milan, elected by them as pope, and burned as a heretic in 1300. She was a relative of the Visconti family, for whom the oldest surviving tarot decks were painted. The suggestion that the Visconti-Sforza card commemorates her is attractive and has been made repeatedly, but the identification rests on inference rather than on any inscription or document. It should be treated as a serious hypothesis, not a fact.
The card was contentious enough that some later printings dodged it. In the Besancon variant of the French pattern the second trump is renamed Junon and given a peacock, and the fifth trump becomes Jupiter, replacing two Catholic offices with Roman gods, a Protestant tidying-up rather than an occult one. Waite's rechristening of her as the High Priestess, with Isis and the veil of the Temple behind it, belongs to a different project again: an attempt to make the card mean something universal rather than something local and awkward.