Crossing fingers to achieve your own good luck or in a display of hopeful solidarity that things go well for someone else is one of the most widely recognized symbols in the Western world. This is partly because of the gesture's long history—although originally, it was not a solo act.
One of the popular explanations for the gesture's origin is that it dates from classical times when Christians were persecuted for their religion, and crossing the fingers was a secret way of invoking the cross.