Many people who are not superstitious actually say things which are superstitious when wishing people well – ‘cross fingers’, ‘best of luck/good luck’ and ‘touch wood’. Cross fingers i guess comes from a religious origin – but is touching wood or knocking on wood a left-over from touching relics, or having religious relics such as a splinter from the cross, or a fragment of a ‘holy’ person’s coffin? Some people argue that it is much older than the Christian religion and goes back to pagan times when some trees were considered sacred, so touching them, or having a bit of wood with you as an amulet would keep you safe. Other people argue that the tree thing is just something made up nineteenth century romantics – and touching wood comes from reaching and touching the door into a place of safety.
There are hundreds of different explanations, from lumberjacks to cattle auctions, coal miners to sailors… here is a link to a selection:
https://www.theguardian.com/notesandqueries/query/0,5753,-22199,00.html
.. and here is something from the Danny Kaye film, ‘Knock on Wood’
http://https://youtu.be/hsxyxn7WuFU
Lois Elsden 
I/d guess that this is pagan in origin – perhaps with Druidic roots.
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