What’s the origin of ‘touch wood’?

There are several theories. According to one, it originates from the pagan belief that good spirits reside in trees such as oak, and hence, touching wood or knocking on wood would help ward off evil spirits. According to an ancient Celtic belief, trees can conduct evil spirits, much like a conductor absorbing heat. Some would knock on wood to thank mythical creatures named leprechauns for bestowing good luck. Also, since Jesus Christ was crucified on a wooden cross, touching a wooden crucifix was, therefore, a form of praying for good luck.