The Charity of Christ Compels Us... 2 Corinthians 5:14
The Scriptures are unanimous in condemning the practice of invoking spirits, sometimes called divination. Consider Deuteronomy 18:10-12:
Let there not be found among you anyone who causes their son or daughter to pass through the fire, or practices divination, or is a soothsayer, augur, or sorcerer, or who casts spells, consults ghosts and spirits, or seeks oracles from the dead. Anyone who does such things is an abomination to the LORD, and because of such abominations the LORD, your God, is dispossessing them before you. You must be altogether sincere with the LORD, your God. Although these nations whom you are about to dispossess listen to their soothsayers and diviners, the LORD, your God, will not permit you to do so.
The Catechism of the Catholic Church teaches (CCC 2116):
All forms of divination are to be rejected: recourse to Satan or demons, conjuring up the dead or other practices falsely supposed to ‘unveil’ the future. Consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers. They contradict the honor, respect, and loving fear that we owe to God alone.
So these practices remain wrong in that they overlook that God is the Lord of life and death and seek to have power in a source who is not God. The evil one and his servants can take advantage of this desire for their own ends, and because of this, it should be avoided. You may think you are speaking with your deceased love ones, but may be actually conversing with a demon in disguise.
All this being said however, we Catholics believe that our prayers are efficacious for those who have “fallen asleep in the Lord.” Our lives here are not the end of the story and so we say “Eternal rest grant to them O Lord,” a special prayer for the dead, asking God for His care for our departed loved ones. We also pray for the forgiveness of their sins in the hope that we will see them once again in the Lord’s coming Kingdom. Christ has destroyed death by His death and therefore we are given the possibility of not being separated from Him even in death, as well as the special obligation to pray for our family and friends who have died.