To cross one's fingers is a hand gesture commonly used to implore God for protection, as well as to wish for good luck. The gesture is referred to by the common expression "keeping one's fingers crossed" or just "fingers crossed" and has also been historically used in order to allow early Christian believers to recognize one another during times of persecution.
Some people, mostly children, also use the gesture to excuse their telling of a white lie. This may have its roots in the belief that the power of the Christian cross might save one from being sent to hell for telling a lie. A similar belief is that crossing one's fingers invalidates a promise being made.
The 1787 A Provincial Glossary, with a Collection of Local Proverbs, and Popular Superstitions by Francis Grose records the recommendation to keep one's fingers crossed until one sees a dog to avert the bad luck attracted by walking under a ladder.
^ abOrange Coast Magazine. Emmis Communications. May 1990. pg. 177. "In early Christian days, a believer confronted by evil or hostile influences implored the power of the Holy Cross for protection by twisting his middle finger over his forefinger and holding the remaining fingers down with his thumb."
^Deborah Aaronson, Kevin Kwan. Luck. HarperCollins Publishers. 2008. pg. 24. "The common explanation for the power and prevalence of this gesture is said to be related to the early Christians. Not allowed to worship Christ in public, his followers showed secret solidarity by crossing their fingers."
^Field Guide to Gestures. Quirk Books. 2003. pg. 201. "Children are a big proponent of this gesture, though they usually use it when telling white lies, believing that having the fingers crossed behind the back makes it okay to fib. Again, this belief may have its origin in Christian crucifix symbolism; calling on the power of the cross might save one from being sent to hell for telling a lie."
^"The Truman Show," 1998. In the film, main character Truman realizes his marriage is a farce when he discovers a wedding photo of his wife with her fingers crossed."
^Jim Jester. Real Israel. 2011. "When they were persecuted in Rome, Christians would secretly come together with the sign of the fish, and they would hold up their crossed fingers, as a Sign of the crossed emblem that had once been on the vestments of the army of Barabbas. It became a custom everywhere, for Christians when meeting, to make the sign of a cross by crossing their fingers."
^"Why do people cross their fingers for luck?". Ask Yahoo!. Yahoo!. October 17, 2002. Archived from the original on 2005-11-24. Retrieved 2013-05-29. "Faith in the power of the Christian cross, therefore, was strong. A cough, a sneeze, or even a mention of a cold (thought to be a sign of the plague) was reason enough to cross yourself. The proper way to make the sign of the cross involves four steps -- touch the forehead, heart, left shoulder, then right shoulder with you right hand. When a suspected witch crossed your path, you could make a cross shortcut by crossing your index and second finger or the index fingers of both hands. This would provide protection and ward off the evil influence."