Saint Joan of Arc said that she had visions from the Archangel Michael, Saint Margaret, and Saint Catherine of Alexandria instructing her to go help the French defeat the English in the Hundred Years War(which I am fine with since the English monarchs by that time were not the true legitimate monarchs). She led the French troops at the Siege of Orleans, and the French won a victory in the battle.
Prior to that, the English were close to winning because in 1420, the queen of France, Isabeau of Bavaria, signed the Treaty of Troyes, which granted the succession of the French throne to Henry V and his heirs instead of her son Charles. Orleans was one of the last cities still loyal to Charles.
And according to Wikipedia, "For generations, there had been prophecies in France which promised France would be saved by a virgin from the "borders of Lorraine" "who would work miracles" and "that France will be lost by a woman and shall thereafter be restored by a virgin".[25] The second prophecy predicating France would be "lost" by a woman was taken to refer to Isabeau's role in signing the Treaty of Troyes."
This shows that there were prophecies that were fulfilled, proving that Joan of Arc's visions telling her to help the French were true and that Christianity is true.