If you're familiar with the Old Testament, you'll know that God takes a very dim view on seeking the dead, demanding instead that we seek Him. Isaiah says it best: "And when they say to you, “Seek those who are mediums and wizards, who whisper and mutter,” should not a people seek their God? Should they seek the dead on behalf of the living? To the law and to the testimony! If they do not speak according to this word, it is because there is no light in them."
This does not fit very well with Catholicism, a pseudo-Christian religion in which the dead, particularly Mary, are sought on behalf of the living. Yet, the law and testimony to which Isaiah referred forbade attempting to contact the dead, even to the point of saying that it is one of the reasons God cast out Israel's prior inhabitants (Deuteronomy 18:11–12). God really does not like us contacting the dead.
But Catholics have a comeback: The saints aren't dead. To them, the very idea that the saints are dead is heresy, in direct contradiction with verses like Matthew 22:31-32, where Jesus says the God of Abraham, Isaac, and Jacob, is not the God of the dead, but of the living. Thus, they say, the saints are alive, and it is as acceptable to seek their intercession as it is to seek a living Christian's.
But there is a major flaw in this interpretation. Abraham, Isaac, and Jacob, from whom Christ Himself argued for the resurrection, are all explicitly said, by God, to have died. Genesis 25:8, "Abraham breathed his last and died...". Genesis 35:29, "Isaac breathed his last and died...". Jacob's death is less explicit, but Genesis 49:33 still says he "...drew his feet up into the bed and breathed his last, and was gathered to his people." We can play this game with numerous other Biblical figures. Moses was definitely saved, yet Deuteronomy 34:5-7 tells us twice that he died. 1 Samuel 25:1, Samuel died.
Samuel is of particular interest here, because he is the only Biblical example of when someone contacted the dead on behalf of the living. Saul sought him out in 1 Samuel 28, and in 1 Chronicles 10:13-14, we see that this is actually the very reason Saul died. So, clearly, the law against contacting the dead is not purely intended for those who died apart from Christ. If, like Samuel, their bodies are without their spirits, James tells us they are dead (James 2:26).
Beyond that, we have an even deeper doctrine that shows saying the saints are not dead is heresy. See, the Christian faith centres around the man whose title it bears: Christ Jesus. Specifically, we are to believe He died and rose again. While He was dead, His spirit did not just poof out of existence. Quite the opposite; by His own words, He went to Paradise during that time (Luke 23:43). He was as alive then as the saints are now, yet it is essential to our faith to affirm yes, He was dead, and on the third day, He rose again.
But did the saints? No. By this point, their bodies may not even be in as good a shape as to be recognisable as human. Dust they are, and to dust they returned (Genesis 3:19). Therefore, by the law of God, by the testimony of Isaiah, those who permit us to seek them on behalf of the living have no light in them. So, what does that tell us about the Catholic Church?