Biblical Basis for Mediums

     I have always been interested in why Christians are so set against spiritual mediums. A lot of Christians don't really seem to believe in psychics or spiritual mediums anyway, but those who do believe in it are very much against it. Why? What does the Bible say against consulting someone who can link you to dead spirits? We see one such dealing with a spiritual medium in 1 Samuel Chapter 28:
"The king said to her, "Do not be afraid. What do you see?" And the woman said to Saul, "I see a god coming up out of the earth." 14 He said to her, "What is his appearance?" And she said, "An old man is coming up, and he is wrapped in a robe." And Saul knew that it was Samuel, and he bowed with his face to the ground and paid homage."
     It seems that, though outlawed by Saul himself at the time, it wasn't uncommon for people to seek out answers from a spiritual medium who consulted the dead. Samuel was quite easily able to seek out just the woman to do the job for him. He didn't know what to do next in his campaign; he was worried. He saw an opportunity to consult his old mentor, Samuel, and he took it.
     Immediately we see one of the reasons why such actions were considered wrong: the spirit of Samuel says, "Then Samuel said to Saul, "Why have you disturbed me by bringing me up?" Apparently, spirits aren't too keen on being called back from whatever they were doing. The news and "advice" that Saul receives isn't really to his liking, though. Samuel tells him that, since he has turned away from obeying God in so many different things, that all of Saul's sons will shortly be dead and spending their time with Samuel in the near future.

     So I've established what the spirits seem to think of being called back to this world, but what about God? Why is it such a big no no? There's plenty of other scripture in the Bible that answers this. In Leviticus it says:"Do not turn to mediums or seek out spiritists, for you will be defiled by them. I am the Lord your God." Furthermore: "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.You must be blameless before the LORD your God." Deuteronomy 18:10-13
     So it's quite clear that God is against these spiritual mediums and He considers it a sin. I think the reason for this is actually quite simple: they're looking to the dead for answers instead of their God. He's constantly pleading with them to turn to Him when they're seeking help and seeking answers and direction from the spirits of those that have already passed is just another example of them relying on man instead of God.

http://uk.answers.yahoo.com/question/index?qid=20070915213849AAe8RzB
http://www.openbible.info/topics/psychic
https://christiananswers.net/q-eden/edn-occult.html