Early necromancy was possibly related to shamanism, which calls upon spirits such as the ghosts of ancestors. Classical necromancers addressed the dead in "a mixture of high-pitch squeaking and low droning", comparable to the trance-state mutterings of shamans. It was widespread among people of Persia, Chaldea, Etruria, and Babylonia. Necromancy was also widespread in ancient Greece from prehistoric times. There are also many references to necromancers, called "bone-conjurers", in the Bible. The Witch of Endor(1 Samuel 28) is the most famous Biblical necromancer. Later Christian writers rejected the idea that humans could bring back the spirits of the dead, and interpreted such shades as disguised demons, thus conflating necromancy with demon-summoning. Interesting enough, some Christians weren't practicing what they preached.
Possibly, literate Europeans were the main forces simultaneously practicing and condemning necromancy. Spells of necromancy are very similar to Christian rites, especially exorcisms. In a Christian exorcism, various demons and spirits are driven away by name, in the name of God, Jesus or the Holy Spirit. It is possible to trace Christian ritual and prayer and its subsequent mutant forms of utility and healing prayer/spells to full-blown necromancy. The main recipe employed throughout the manual in the necromancy sorcery uses the same vocabulary and structure utilizing the same languages, sections, names of power alongside demonic names.
Necromantic spells were mainly illusory or utility spells. Modern scholarship suggests that most were written with hopes that their utility would prove to be useful in acquiring a feast, horse, cloak of invisibility or perhaps just notoriety among others in the necromancy practicing clergy. The nature of these spells lend themselves to being understood as underground clergy members deviantly indulging in unlawful pleasures. The probable reason that these renegade so-called Necromancers were dabbling in the dark arts is that the evolution of "natural" magic and "spiritual" magic was slow.
In the wake of these inconsistencies of judgment, necromancers, sorcerers and witches were able to utilize spells with holy names with impunity, as biblical references in such rituals could be construed as prayers as opposed to spells.
Modern séances, channeling and Spiritualism verge on necromancy when the invoked spirits are asked to reveal future events. Necromancy may also be dressed up as sciomancy, a branch of theurgic magic. Necromancy is not practiced in Neo-pagan Witchcraft, but it is practiced in Voodoo. Necromancy, more than any other magical system, is responsible for the reputation of occult cults and sects holding graveyard rites and performing the infamous "blood sacrifice".Source: Wikipedia