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 in part because of the gesture's long history—although originally, it was not a solo act.
There are two main theories regarding the origins of finger-crossing for luck. The first dates to a pre-Christianity Pagan belief in Western Europe in the powerful symbolism of a cross. The intersection was thought to mark a concentration of good spirits and served to anchor a wish until it could come true. The practice of wishing upon a cross in those early European cultures evolved to where people would cross their index finger over that of someone expressing a wish to show support. Eventually, wish-makers realized they could go it alone and impart the benefit of a present cross to their wishes without another person's participation, first crossing their two index fingers and finally adopting the one-handed practice we still use today.
The alternate explanation cites the early days of Christianity, when practitioners were persecuted for their beliefs. To recognize fellow Christians, people developed a series of hand gestures, one of which involved forming the ichthys, or fish symbol, by touching thumbs and crossing index fingers. This theory doesn't fully explain how luck initially became associated with the gesture, but it does posit that the solo finger cross developed during the bloody Hundred Years War by soldiers eager for anything that might curry God's favor.