The practice of taking over existing cultural holidays and "Christianizing" them is something the Church has done practically since its inception. In a well-known speech to the Athenians in the 1st century, the Apostle Paul declares the deity they called "Agnostos Theos," or unknown god, was actually the Hebrew god, whose name was too holy to say. (Acts 17:22-31) There was a temple in Athens dedicated to this god, but it wasn't viewed so much as a deity itself but as a way the Greeks could cover their spiritual hindquarters in case there was a god that existed that they didn't know about and didn't want to inadvertently insult. Paul, being an educated scholar, used his understanding of the Greek culture and literature to sway his listeners to a different spiritual concept, but also using one that was already embedded in the culture. Conversion to Christianity, in Paul's experience, came as a dramatic "Aha!" moment in which he was struck blind for a time. When he could see again, it was as if he saw everything in creation in a new light with a new perspective. Therefore, the effort to convert others to Christianity involved, at least early in the history of the movement, persuading them to see their existing world through the lens of a Christian perspective. That Sun God you worship? That's the Son of God. The death and rebirth you witness each year on the earth? That symbolizes Christ's death and resurrection, and so on and so forth. It wasn't until the Church became a civil authority with the conversion of the Roman Emperor Constantine that it had the official power to not only persuade but demand certain feasts and festivals honor Christian concepts rather than pagan. Constantine himself was Roman and did not cease to be Roman after his conversion. Rather, he credited his successes to the Christian High God but continued to honor the Roman deities as well. In fact, he instructed that Christians and non-Christians alike were to observe the venerable day of the sun. This edict would lead to the transforming of the pagan holiday of Yule into Christmas, but Constantine was happy with it as it was.
Meanwhile, as we don our costumes, attend parties and trick-or-treat, we are participating in an amazing cross-cultural, cross-spiritual event, whether we recognize it or not. We're paying homage to Death and, by extension, to Life. We're facing our fears, whistling in the dark, and honoring our ancestors who have all had to face the same ultimate end. We all participate in the dance macabre.