@simrin-- Actually, that has a similar origin with crossing fingers for luck. They both have origins in Christianity as the article mentioned (aside from the Hundred Years War Story).
As far as I know, during the time that Christians were being persecuted, if they were questioned about their religious beliefs, they would lie saying that they were not Christian. But while they lied, they would cross their fingers behind their back to nullify what they were saying.
Christians would also cross their fingers to show their belief in Jesus who was crucified on a cross.