The roots of Halloween are in Catholicism, formed by Constantine around 325 AD. The word Halloween is probably a derivative of "All Hallows Eve," a spin-off of "All Hallows Day" or "All Saints Day," which is a day to honor Saints.
     In the fifth century AD, in Celtic Ireland, the New Years day celebration called Sam Hain was at the end of summer, on October 31. The theory was that on October 31 the spirits of those who died in the proceeding year would arise from the dead, and look for bodies to inhabit throughout the next year. The Celtics believed the rules of time and space ceased during this day, and spirits could mix with humans.

     The Roman Catholics adopted the Celtic practices, but changed the deity from the spirit world to Pamona, the goddess of fruit and trees. Her symbol was the apple, from which possibly came the Halloween practice of bobbing for apples. Time passed, and other gods and goddesses became part of the tradition. Some, like "Hecate," who was a favorite of Zeus was friendly, while others, like vampires, invoked fear. The costumes villagers wore depicted these god's and goddesses. 
     In the 1840's, as a variety of people immigrated to the USA they brought their customs with them. Today's trick or treating probably derived form European customs on All Soul's Day. Christians would go around to the houses and ask for "soul cakes". The more soul cakes the more prayer to release friends and relatives from purgatory. The idea of lit jack-o-lanterns more than likely came from the Irish, and their belief that spirits carried them on All Saints Day to light their way.