Why the letter X means Kiss

In the Middle Ages, when few could write, the iliterate signed documents with a cross to denote their name. The cross was seen as a respectable mark for anyone to make because it was the Christian symbol. After a document had been signed, it was common to kiss the page it was writen on, to show it was being signed in good faith. At some point over the years, the kiss and the "X" became confused, and now we still sign off letters or text messages with an X to denote a kiss.