Trick-or-treating can be traced back to the Festival of Samhain (See my last post, The Pagan Roots of Halloween: How It All Began). By offering gifts of fruits and nuts, the people hoped to appease the wandering spirits of their dead relatives. They would place plates of their finest food on their doorstep in hopes that the spirits wouldn't destroy their crops or cattle. If the living did not provide "treats" for the spirits, they believed the spirits would play a "trick" on them.
Some of the people, though, took on a mischievous humor to their fear and would play a trick or practical joke, and then blame it on the spirits. This night also became know as "Mischief Night," where people could play any kind of trick - without fear of being punished.
So, the idea of trick-or-treating came from "treats" being left out for spirits, who hopefully wouldn't play any "tricks" on them in return. But the people were slightly worried: If the souls of their dead loved ones were roaming about, couldn't the souls of everything else - nice or not - be roaming about too? They didn't just have their relatives' spirits to worry about, they believed the evil spirits were active on this night also.
Since they believed evil spirits roamed about after dark on Halloween, they sought to disguise themselves. Wearing costumes, they thought, would scare away evil spirits or keep themselves from being recognized. By wearing masks, disguises, and blackening their faces with soot, they hoped to blend in unnoticed with the demons.
Besides being used to hide one's identity, a mask (they believed) could also be used to make spiritual connection with the nature represented in the mask. For instance, a Celtic Druid participating in ceremonial rituals wore animal heads and skins, believing they would acquire the strength of the particular animal. Because they thought the mask had its own magical powers, they would wear it to try to absorb the power of the animal represented in the costume.
The night of Samhain was a night of fear. Although the Celtics' belief in the spirits' tricks was mainly pagan ideas, the reality of evil is real. Demons are real. Witchcraft is real. Evil is something that should not be celebrated - no matter how light-hearted or fun you make it seem.
And do not be conformed to this world, but be transformed by the renewing of your mind...
1 Peter 5:8
Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour.