In the Middle Ages, churchmen insisted that knocking on wood was part of their tradition of prayer, since Christ was crucified on a wooden cross. But the tradition of knocking on wood actually started several thousands of years earlier, with a different deity. Bot Native Americans and ancient Greeks developed the belief that oak trees were the domain of an important god. By knocking on an oak, they were communicating with the god and asking for his forgiveness. The Greeks passed their tradition on to the Romans and it became part of European lore. The oak’s power was eventually transferred to all types of wood.