'x' marks the spot

X

originated as the Phoenician letter samech, which has been translated as prop, support, post or a fish. X was related to the Greek xei, which sounded like "ks". The Romans adopted the Greek xei but used the form of the Greek chi. X was the last letter of the Roman alphabet until around 50 AD. Several authorities — Ben Jonson among them — consider X to be a less than legitimate letter because the ks would replaced its sound so easily. Though X is the least used letter in English, its a well known symbol of Christ (Xmas), was used as a signature by illiterate people throughout history, and has always “marked the spot”.
http://www.alphabetandletter.com/X.html

X is Christ
x is the unknown
X is the way to the eternal Treasure that we continue to search for, digging up fields and giving all that we have to own that Treasure