If you only know one heretical practice of the Catholic Church, it's probably the worship of Mary. The trouble with witnessing to Catholics, however, is that most of them will not admit this. Catholics distinguish between worship and what they call "veneration". In their view, worship is that which is owed only to God, whereas veneration is merely showing people the respect they deserve.
If you want to find out about Catholic doctrine, by far the best place to go is the Catechism of the Catholic Church. Published in 1992, and firmly endorsed by Pope John Paul II, the Catechism of the Catholic Church "...is offered to every individual who asks us to give an account of the hope that is in us (cf. 1 Pt 3:15) and who wants to know what the Catholic Church believes." (1). While I have yet to receive a consistent answer on whether Catholics believe the Catechism of the Catholic Church (henceforth referred to as either "CCC" or "The Catechism") is infallible, it seems odd to suggest that a book promulgated by a Pope specifically to tell every individual who wants to know what the Catholic Church believes would be in some way a poor representation of what the Catholic Church believes. Therefore, whatever CCC says about Mary should be considered true Catholic teaching.
The problem Catholics have is that while they suggest they do not worship Mary, the Catechism alone elevates Mary not only to the level of goddess, but even portrays her as effectively being a second Christ.
Even before we get to the really messed up stuff, the Catechism elevates Mary above the level of a normal human. CCC 491, for example, claims "...Mary, "full of grace" through God, was redeemed from the moment of her conception...". CCC 493 further states "...By the grace of God Mary remained free of every personal sin her whole life long....".
Already we hit some very serious problems. The Bible says absolutely nothing about God preserving Mary from any sin. This, in itself, is problematic for Catholic Mariology. Amos 3:7 tells us that the Lord does nothing without revealing His secret to His prophets. We see this amply demonstrated throughout the Bible. In the Old Testament, Jesus' life, ministry, death, resurrection, and even second coming, are all at least hinted at. We see that He would be born of a virgin (Isaiah 7:14) in Bethlehem (Micah 5:2), that He would be betrayed by a friend (Psalm 41:9), that He would be rejected by His mother's children (Psalm 69:8), that He would bear the sins of the world (Isaiah 53:5), and even implications of His resurrection (Isaiah 53:10-12). If we were to list all the prophecies of Christ in the Old Testament, the list would exceed even the references to Mary in the Catechism. Yet, the references to Mary in the Old Testament are minimal, and in no way reflect Catholic Mariology. If the Lord does nothing without telling His secret to the prophets, yet He told no prophets He would do something special with Mary, it makes sense that the Lord did nothing special with Mary.
The New Testament further confirms this, telling us that all have sinned (Romans 3:23), which, of course, would include Mary. In fact, in 1 John 1:8-10 when John says anyone claiming to say they have no sin is self-deceived, and we make God out to be a liar, proving His word is not in us, he does not note a single exception. The only exception ever cited is Jesus. No Biblical author ever considered Mary to be sinless, nor does it appear Mary herself ever made such a pride-filler error. She rejoiced in God, her savior (Luke 1:47), and even made a sin offering shortly after Jesus' birth (Luke 2:22-24, cf. Leviticus 12:7-8). Catholics will argue that this is merely to "fulfil all righteousness", similar to Jesus' baptism (Matthew 3:15), but unlike with Jesus' baptism, this is nowhere justified in the text. It is an interpretation invented entirely to defend the Marian dogmas. This is blatant eisegesis, no better than when Mormons approach Isaiah's declaration that there is one God by saying there is only one God on whom we need to focus, whereas they believe there is an infinite regression of Gods.
So already Catholic Mariology is off to a bad start, elevating Mary above other humans. In reality, there was nothing intrinsically special about her, just as with all God's chosen. As the saying goes, God does not call the qualified, He qualifies the called. No doubt, being chosen to bear the Messiah is a great blessing. Nevertheless, Christ Himself tells us it is a greater blessing to hear and obey the word of God (Luke 11:27-28). It makes no sense for Catholics to therefore justify their "honoring" of Mary with verses like Luke 1:48.
But as the many idols of Mary dotted around the Catholic world prove, Catholicism doesn't stop there with Mary. Not only does it teach that Mary was free of sin, but also that she did such meritorious works that she is able to absolve Catholics of punishment for their sins through indulgences. Paragraphs 1471-1479 of the Catechism describe "indulgences", which are defined as "...a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints." (CCC 1471). According to paragraph 1478, "An indulgence is obtained through the Church who, by virtue of the power of binding and loosing granted her by Christ Jesus, intervenes in favor of individual Christians and opens for them the treasury of the merits of Christ and the saints to obtain from the Father of mercies the remission of the temporal punishments due for their sins." In paragraph 1477, we are told that the Treasury of Merits, while Christ's merits keep it infinitely supplied, "...includes as well the prayers and good works of the Blessed Virgin Mary. They are truly immense, unfathomable, and even pristine in their value before God....".
Once again, this is nowhere justified in scripture. The Treasury of Merit, much like the realm of Purgatory in which indulgences can reduce your time (CCC 1472), is a completely fictitious concept. The idea that the Catholic Church has the ability to draw on the prayers and good works of Mary in order to remit the temporal punishment for sin is a novel, and frankly blasphemous one.
The first problem is that neither Mary's works, nor the rest of the saints, are sufficient to save at all. In fact, Isaiah tells us that even our righteous acts, because we are sinners, are filthy rags to God (Isaiah 64:6). Christ, and Christ alone, is free from the stain of sin, and thus His works alone save us. He became sin who knew no sin that we may become the righteousness of God in Him (2 Corinthians 5:21). Because of this, there is no condemnation for those who are in Jesus (Romans 8:1), and He is able to save us to the uttermost, because He, not a priest, not a bishop, and not a Pope, lives forever to intercede for us (Hebrews 7:25). He is completely alone in this (1 Timothy 2:5).
"Not so!" saith the Catechism. ""This motherhood of Mary in the order of grace continues uninterruptedly from the consent which she loyally gave at the Annunciation and which she sustained without wavering beneath the cross, until the eternal fulfilment of all the elect. Taken up to heaven she did not lay aside this saving office but by her manifold intercession continues to bring us the gifts of eternal salvation .... Therefore the Blessed Virgin is invoked in the Church under the titles of Advocate, Helper, Benefactress, and Mediatrix."" (CCC 969). Mediatrix.
Mediatrix.
According to scripture, "...there is one God, and one mediator between God and men, the man Christ Jesus; Who gave himself a ransom for all, to be testified in due time." (1 Timothy 2:5-6). But according to Catholicism, Mary continues to bring us the gifts (ironic, a gift you must pay for...) of eternal salvation, earning her the title of Mediatrix.
Through the abominations we have discussed so far, we see that while the real, historical Mary was very humble, the Catholic Mary seems more and more like Christ. Conceived without sin, just like Christ. Remaining free of sin, just like Christ. Being exceedingly meritorious, just like Christ. Even interceding between man and God, just like Christ. It turns out, the Catechism even goes as far as to say Mary is Queen of all things, just as Christ is King over all things! ""Finally the Immaculate Virgin, preserved free from all stain of original sin, when the course of her earthly life was finished, was taken up body and soul into heavenly glory, and exalted by the Lord as Queen over all things, so that she might be the more fully conformed to her Son, the Lord of lords and conqueror of sin and death." The Assumption of the Blessed Virgin is a singular participation in her Son's Resurrection and an anticipation of the resurrection of other Christians: In giving birth you kept your virginity; in your Dormition you did not leave the world, O Mother of God, but were joined to the source of Life. You conceived the living God and, by your prayers, will deliver our souls from death...." (CCC 966).
To a faithful Christian, this should not even seem sensible, much less sound Christian doctrine. To suggest that Mary is Queen over all things is to elevate her to the level of Christ, as if calling her Mediatrix doesn't already do this. In a sense, it already admits this, claiming this is "so she might be more fully conformed to her Son". And of course, the paragraph just has to include yet another suggestion that Mary will deliver our souls from death.
And it still doesn't stop there. In the sections on the second commandment, CCC stops just short of calling Mary God. Paragraph 2146 tells us "The second commandment forbids the abuse of God's name, i.e., every improper use of the names of God, Jesus Christ, but also of the Virgin Mary and all the saints."
It is actually rather ironic that the Catholic Church should refer to the second commandment this way, as the Catholic Church actually lists the 10 commandments very differently. Whereas Jews (2) and "Protestants" would have the prohibition of taking the Lord's name (not Mary's) in vain listed as the third commandment, the Catholic Church lists it as the second, splitting what would normally be the 10th commandment into 9 and 10. Now, the ordering itself is not necessarily problematic. Officially, there is no order, nor are there even verse divisions. Instead, the 10 commandments are found in Exodus 20:2-17. So, what's the problem?
The problem is that Catholic listings of the 10 commandments tend to completely ignore the content of Exodus 20:4-6: "Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me; And shewing mercy unto thousands of them that love me, and keep my commandments."
Graven images. Kind of like the stone Mary displayed in the header image. The traditional Catholic listing of the 10 commandments, memorised by Catholics around the world, so brazenly ignores the command against graven images that many ex-Catholics cite this mistreatment of scripture among their reasons for leaving the Catholic Church.
But "aha", the Catholic may reply, "you're misquoting our sources!" See, the Catechism, in anticipation of this objection, seeks to justify the veneration of idols in paragraphs 2129-2132. It argues that God obviously cannot be against all graven images, as He had already prescribed such images in the form of the Bronze Serpent, the Ark of the Covenant, and the Cherubim.
What it neglects to inform the reader is that, while these were indeed prescribed, they were actually taken from us precisely to prevent veneration. As paragraph 2129 hints at, Deuteronomy 4:15-19 talks a lot about graven images, but not merely as the prohibition of representations of God. Indeed, any figure in the likeness of male or female (v16), any beast of the earth or fowl of the air (v17), or any creeping thing or fish of the water (v18), lest the Israelites should be driven to worship them (v19). This, in fact, is the whole reason the aforementioned bronze serpent was destroyed (2 Kings 18:4).
What, then, is the logical response to Catholics constantly bowing down to these statues of Mary? Regardless of what the 7th ecumenical council of Nicaea may have said in 787 A.D., these statues should be immediately demolished, not encouraged. Yet, they are, because "Religious worship is not directed to images in themselves, considered as mere things, but under their distinctive aspect as images leading us on to God incarnate. the movement toward the image does not terminate in it as image, but tends toward that whose image it is." (CCC 2132).
Catholicism is so filled with such repugnant doctrines as described in this article that if I had enough patience and coffee, I could easily have extended it a hundredfold. I believe, however, that by merely comparing these few extracts from the Catechism of the Catholic Church to the divinely inspired word of God, I have sufficiently demonstrated that the Catholic Church has a very blasphemous view of Mary. The way she is presented within this Pope-endorsed book is completely unacceptable, so close to worship that the word need not be used to prove the concept. Catholics can call it veneration all they want, but a rose by any other name has some very nasty thorns. Thankfully, Jesus, not Mary, wore those thorns as He hung from the cross, meaning even the most dedicated, dyed in the wool Catholic can repent of "venerating" Mary and receive the remission of their sins, not by accessing the mythical Treasury of Merit, but instead because the Risen Lord ever lives to intercede for us. Catholics, repent, because just as the Jews angered God by worshipping the Queen of Heaven (Jeremiah 7:18; 44:17-25), so do you.
References
1. Pope John Paul II, Apostolic Constitution, Fidei Depositum on the Publication of the Catechism of the Catholic Church, October 11th 1992 (link).
2. Rabbi Ronald H. Isaacs, The Ten Commandments, My Jewish Learning (link).