Spiritism is the belief that the living can contact the dead. Through a person called a "medium," the living are able to speak to someone in the unseen world. The Bible strongly condemns this practice.

Forbidden In Scripture

The Bible is very clear that the living should not attempt to contact the dead. In ancient Israel those who practiced such things were put to death. Moses wrote.

You shall not permit a sorceress to live (Exodus 22:18).

The Law also said.
  

You shall not eat anything with the blood, nor shall you practice divination or soothsaying . . . Give no regard to mediums and familiar spirits; do not seek after them, to be defiled by them: I am the Lord your God. . . And the person who turns after mediums and familiar spirits, to prostitute himself with them, I will set My face against that person and cut him off from his people . . . A man or a woman who is a medium or a wizard shall be put to death; they shall be stoned to death, their blood is upon them (Leviticus 19:26,31; 20:6,27).

1.  Do Not Imitate Heathen

The people of Israel were warned, before they entered into the Promised Land, not to imitate the ways of the heathen.

When you enter the land the LORD your God is giving you, do not learn to imitate the detestable ways of the nations there. Let no one be found among you who sacrifices his son or daughter in the fire, who practices divination or sorcery, interprets omens, engages in witchcraft, or casts spells, or who is a medium or spiritist or who consults the dead. Anyone who does these things is detestable to the LORD, and because of these detestable practices the LORD your God will drive out those nations before you  (Deuteronomy 18:9-12).

The levitical laws forbade any attempted contact with the dead.

The Prophets

The prophets also spoke out against such practices. Isaiah wrote.

Now if people say to you, "Consult the ghosts and the familiar spirits that chirp and mutter; should not a people consult their gods, the dead on behalf of the living, for teaching and for instruction?" Surely, those who speak like this will have no dawn! (Isaiah 8:19,20).

2.  Lying Spirits

There is also the possibility that lying spirits may speak to people.

Now the Spirit expressly says that in later times some will renounce the faith by paying attention to deceitful spirits and teachings of demons (1 Timothy 4:1).

3.  Wrong  Source

The Bible forbids talking to the dead, not because it is necessarily a hoax, but because it relies on supernatural guidance that comes from a source other the God. Whatever the ultimate source may be, their desire is to lead people away from the living God.

Summary

We can make the following points about spiritism, or trying to contact the dead.

1.  The Bible forbids consulting spirits of the dead.

2.  Much of the phenomena of spiritism can be attributed to deception.

3.  Seeking supernatural guidance from a source other than the Lord can cause the person to be led astray.