One of the first images in “Frida Kahlo: Making Her Self Up” at the Victoria and Albert Museum in London shows the artist on the day of her First Holy Communion in Mexico City. She is wearing the traditional white outfit, with a long veil trailing down her back, and she is kneeling at a prie-dieu. But her head is resting on her hand in a way that makes her look quizzical about it all: and on the back, a few years afterwards, she has written the word Idiota to reference the fact that, by this stage, she has decided that Catholicism is so much bunkum.