"Hail Mary, full of grace. Our Lord is with you. Blessed are you among women, and blessed is the fruit of your womb, Jesus. Holy Mary, Mother of God, pray for us sinners, now and at the hour of our death. Amen."
What you have just read, according to the Bible, is an abomination. Calling upon the dead is utterly forbidden in the law, and is called an abomination (Deuteronomy 18:10-12). It is an abomination so great that it is actually listed as one of the reasons God cast out the prior inhabitants of Israel. Following this tradition, Isaiah writes "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." (Isaiah 8:20).
We see quite clearly how much God hates when people contact the dead, especially His people, whom He requires to ask Him instead. Yet, as you can see, Catholicism is full of it. Thus, reckoning by the scriptures, Catholicism is an abomination.
Catholics have two main responses. The first is to claim that Mary and the saints aren't actually dead, since they're in Heaven. The second is that they're not actually praying to saints, but rather, they're asking the saints to pray for them because they have more direct access to God. You know, being dead and all...
Of course the saints are dead. 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. We could go on for ages.
Samuel is an especially important example, because Saul contacted him for intercession when God stopped answering him. Samuel's response? "So why do you ask me, seeing the Lord has departed from you and has become your enemy?" (1 Samuel 28:16). Contacting Samuel for intercession was both evil, and futile, and for our point, shows that there is literally no distinction between contacting dead believers and unbelievers. When someone has left this realm, contacting them is unacceptable.
But let's just suspend that fact for a moment. Let's ignore the constant references to believers' deaths in scripture and pretend God just raptures His people. Is it true that they have a more direct access to God? Turns out, no. Hebrews 4:16 says "Let us therefore come boldly to the throne of grace, that we may obtain mercy and find grace to help in time of need." The dead in Christ may well be in His presence, but we are just as capable of speaking to Him, and He will respond to us. If we imagine Mary was so omniscient as to be able to hear the millions of people praying to her, she would not be able to influence God to act any more than you, or your living Christian friend could.
And so I restate, Catholicism is an abomination. A number of Old Testament heresies, including communion with the dead, can be found in that false Church. Christians, therefore, should have nothing to do with it. Being in the world does not involve being in a worldly Church. Catholics need to repent.