What is the origin of this strange ritual to show honesty? Is it found outside the US and (presumably) Canada? Is it a religious reference to being protected by the cross, perhaps?
A form of it is found in France, too : “Croix de bois, croix de fer, si je mens, je vais en enfer” (Cross of wood, cross of iron, if I lie may I go to Hell). While the wood and iron thing is probably just a bit of nonsense to make it rhyme and have the right meter/rythm, it was traditional to do the sign of the cross twice when saying this.
As to the why, no idea. WAG : doing the sign of the cross draws God’s/Jesus’/the saints’/the angels’ attention to you, so if you do lie right after doing so, smiting is more likely to ensue.
I was going to WAG it is like the opposite of saying “If I am lying, may lightning strike me…”
“Cross my heart, hope to die, stick a needle in my eye” is a fairly common British expression to swear to honesty.
Very common in San Diego when I was little, too.
I can only guess, but I suspect it comes from Catholic genuflection, where the person crosses his heart. Not being a religious person, I don’t know exactly what it means, but I have it in my head that it’s something along the lines of ‘In the name of the Father, the Son, and the Holy Ghost’. (I’m sure someone will come along to correct/inform me shortly). So kids (primarily) swearing by crossing their hearts seems to be that they are swearing an oath on the name of the Deity.
Are you asking about the cross, the heart, or the honesty? The cross is religious; it resembles the sign of the cross. The heart part is for the same reason we say “bottom of my heart”, “heart on a sleeve”, and “cold-hearted”. The honesty part is because it’s an oath, punishable by some righteous holy action.