In early Semitic writing the letter X was called samekh, meaning “fish.” It was not used as a letter but as a picture symbol for a fish. The Greeks first used X as a letter. The X became associated with the crucifixion and symbolized truth.
Illiterate persons signed documents with an X as an oath that the content is true. Even some kings – although they could write – used the X as a sign of truth and commitment. This symbol of truth eventually became associated with kisses.
So, when you sign a card that says “I love you” off with Xs, not only are you sending kisses but you’re saying, “This is the truth!”