Leviticus 20:27 says clearly, "a man or a woman who is a medium or spiritist must be put to death. You are to stone them; their blood will be on their own head." Equally emphatic is Deuteronomy 18:10-11: "Let no one be found among you who sacrifices a 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."
Lest we think these texts are just examples of Old Testament stringency that we do not find in the New Testament, Paul condemns idolatry and sorcery or witchcraft, saying in Galatians 5:20 that no one who practices such things will inherit the Kingdom of God. Equally clear is Acts 19:19, in which Greeks who had practiced sorcery publicly burned their spell books and scrolls containing magical incantations, frightened by the evidence of the power of Jesus they saw in Paul and others.
The biblical tradition sees mediums and necromancers (those who call up the dead) as problematic because they seek special knowledge, usually of the future, from an inappropriate source-someone other than God, such as a deceased person or evil spirit. The Bible disapproves of people who fail to: 1) seek God's will in God's revealed Word; or 2) be satisfied by what they have learned from reading God's Word; or 3) accept the biblical God in favor of polytheism or other non-biblical forms of religion. This latter reason is why Paul associates sorcery of any sort with idolatry.
But what about astrology? Reading your daily horoscope is miles apart from true astrology. Consulting the stars to learn something about your fate or about the future has been going on almost as long as recorded human history. The basic notion that undergirds it is that somehow important future events are revealed in the stars, or that the heavenly patterns which existed on a person's birth date help determine their fortune, fate, character and personality traits. The ancients believed the stars revealed the divine will because they also believed the stars were themselves beings--the heavenly hosts, angels, and the like. They did not view stars as inanimate matter in the sky that somehow controls earthly relationships or events. Ancient astrologers were polytheists who believed that some of the beings they worshipped could be seen in the sky at night, and so could be studied, and the will of the gods could be discerned.