Starters:
Leviticus 19:31 Regard not them that have familiar spirits, neither seek after wizards, to be defiled by them: I [am] the LORD your God.
Deuteronomy 18:[9] "When thou art come into the land which the LORD thy God giveth thee, thou shalt not learn to do after the abominations of those nations. [10] There shall not be found among you [any one] that maketh his son or his daughter to pass through the fire, [or] that useth divination, [or] an observer of times, or an enchanter, or a witch, [11] Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. [12] For all that do these things [are] an abomination unto the LORD: and because of these abominations the LORD thy God doth drive them out from before thee. [13] Thou shalt be perfect with the LORD thy God. [14] For these nations, which thou shalt possess, hearkened unto observers of times, and unto diviners: but as for thee, the LORD thy God hath not suffered thee so [to do]."
Isaiah 8:19 And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead?
Now what does the Lord say about these people? 2 Chronicles 33:6 "And he caused his children to pass through the fire in the valley of the son of Hinnom: also he observed times, and used enchantments, and used witchcraft, and dealt with a familiar spirit, and with wizards: he wrought much evil in the sight of the LORD, to provoke him to anger. " Hmmm. These things provoke the Lord to anger? To what extent?:
Leviticus 20:6 And the soul that turneth after such as have familiar spirits, and after wizards, to go a whoring after them, I will even set my face against that soul, and will cut him off from among his people.
Leviticus 20:27 A man also or woman that hath a familiar spirit, or that is a wizard, shall surely be put to death: they shall stone them with stones: their blood [shall be] upon them.
1 Chronicles 10:13 So Saul died for his transgression which he committed against the LORD, [even] against the word of the LORD, which he kept not, and also for asking [counsel] of [one that had] a familiar spirit, to enquire [of it];
God specifically says not to have part in the works of wizards (necromancers included in that). He specifically states elsewhere that those who practice their arts should be killed. And he further links wizards and those with familiar spirits to communicating with the dead.
Now, Want to tell me how you have the right to teach anyone that it is not forbidden? There is more where that comes from. Care to guess how many times prohibitions against wizards and those with familiar spirits are mentioned in the OT alone? And would anyone care to eat some words regarding Deuteronomy yet? Three in a row in Deuteronomy verse 18:11 all having to do with communing with the dead and so linked and charged within the text of the Bible. All three arts condemned and practicers of such arts to suffer death! And God saying in Isaiah 'shouldn't they be turning to Me concerning the dead?'
Wow, I wonder why God's word also speaks against the worship of the dead, lest we forget a certain couple at the heart of the Mithraic rites who are mentioned in the Bible.