Crossing our fingers for luck predates Christianity and originally involved two people.
In the pagan ritual, a close friend placed his or her index finger over the index finger of the person making the wish in order to help trap the wish at the centre of a perfect cross, which is where benevolent spirits lived.
To ensure the wish stayed in place and on the wisher’s mind, it was often tied to the finger with string, a practice that eventually evolved into a memory aid.