I think X was actually for the Greek letter Chi - A footnote on Xpn used in 1640's states This is an early form of the surname Christian, from the Greek X P I C T O C = Christ. The monogram composed of the first three letters X P I of the Greek word became the distinctive symbol of the Christian faith and was so used in early monuments.
see http://www.ee.surrey.ac.uk/Contrib/manx/jmmuseum/d212.htm