As a young man I explored as many of my County's mediaeval churches as I possibly could, and soon became fluent in the iconography of the surviving painting and sculpture. A man walking across a stream with someone on his back was St Christopher, of course, and a chap tied to a tree with arrows sticking out of him St Sebastian, except when he was headless or depicted with a wolf at his feet in which case he was St Edmund. St Bartholomew was usually holding his own flayed skin, St Catherine tied to a wheel, St Michael with a dragon and so on. As it was designed to be deciphered by simple peasants I had little problem with it.
Later, a basic classical education helped understand post-Renaissance painting, which to the uninformed seems mainly to be acres of naked pink women with wispy bits of gauze and enigmatic smiles. Lots of naked pink women usually meant it was The Rape of the Sabine Women, and a single naked pink woman standing in water The Birth of Venus. The art of the counter Reformation brought naked pink rent boys, dressed up by Caravaggio as, well, anything he could think of, really. But his saints got more complex. St Jerome had the skull and book, but only a hint of the nimbus or halo that shouted 'Saint!' and in The Calling of St Matthew he had to include three pointing fingers including Christ's to indicate it was the hungover rent boy and not the bearded elder who was St Matthew.
Still, with a bit of knowledge you could work your way through it. Sister Wendy, now 82, makes the point in the Telegraph that the young are losing out in understanding the art that fills our great galleries and churches because they know neither their bibles nor the classics. This is undoubtedly true, but not a matter to despair.