Praying to Mary is idolatry. It's pretty cut and dried. There are numerous verses in the Bible that show it.
Does the Roman Catholic Church promote idolatry? According to the scripture and its own practices, yes it does.
Roman Catholics have an extremely high view of Mary and have attributed some astounding characteristics and abilities to her, such as her interceding for us, atoning for us, and delivering our souls from damnation (documented below). Protestants consider this elevation to be not only extreme but also idolatrous since they attribute to Mary what should only be said of God. Roman Catholicism responds by saying it venerates Mary and gets its doctrines on her from Sacred Tradition, which is an "inanimate thing passed from hand to hand,"1. Alright, so which is it, the sin of idolatry or the blessing of veneration?
The Roman Catholic church teaches there is a difference between idolatry and veneration. It says,
"Idolatry etymologically denotes Divine worship given to an image, but its signification has been extended to all Divine worship given to anyone or anything but the true God . . . An essential difference exists between idolatry and the veneration of images practised [sic] in the Catholic Church, viz., that while the idolater credits the image he reverences with Divinity or Divine powers, the Catholic knows "that in images there is no divinity or virtue on account of which they are to be worshipped, that no petitions can be addressed to them, and that no trust is to be placed in them."2
The Roman Catholic Church has defined idolatry far too narrowly in its favor. Let's consider two things.
Divine Worship
First, notice that it says "divine worship" should be given only to God. This works out to mean that other forms of worship to Mary are okay as long as it isn't "divine worship." But what is "divine worship?" After searching through the Vatican Website3, the Catholic Encyclopedia4, the entire Council of Trent5, Vatican II6 and the Catechism of the Catholic Church,7, Fundamentals of Catholic Dogma,8 and the Catholic Dictionary9 the best I could find was, "Now the divine worship is directed to the glorification of God."10 That is a nice and brief comment, but it isn't descriptive enough. Why is it that in all these sources Divine Worship is not defined? Perhaps it is because offering a strict definition might make them vulnerable to criticism concerning their worship of Mary.
Catholicism says that divine worship is for God only, yet it also says it is okay to bow down before a statue of Mary, pray to Mary, believe that Mary delivers us from death, believe Mary atoned for us, etc., as long as you don't give her "divine worship." In other words, you can do almost anything worship-wise to Mary as you would to God - just don't call it "divine worship."
Exodus 20:5 and "worship"
The Hebrew word for "worship" in Exodus 20:5 (the Ten Commandments) is "shachah" שָׁחָה. Take a look at what the Strong's Enhanced Lexicon says about the word.
"172 occurrences; AV translates as “worship” 99 times, “bow” 31 times, “bow down” 18 times, “obeisance” nine times, “reverence” five times, “fall down” three times, “themselves” twice, “stoop” once, “crouch” once, and translated miscellaneously three times. 1 to bow down. 1a (Qal) to bow down. 1b (Hiphil) to depress (fig). 1c (Hithpael). 1c1 to bow down, prostrate oneself. 1c1a before superior in homage. 1c1b before God in worship. 1c1c before false gods. 1c1d before angel."11
Bowing down is part of the act of worship - as you can see above. Clearly, Roman Catholics who bow down before statues of Mary (and others) are risking breaking the commandment not to worship other gods and not to bow down before idols. The Catholic Church responds by saying that as long as it isn't divine worship given to Mary, it is okay. But this is nothing more than a word game. They do the same thing to Mary that they would to God and excuse it by saying that it isn't divine worship. The funny thing is that God doesn't make the same distinction as the Catholics do. God says don't do it. Don't bow down before images (Lev. 26:1).
Idolatry
Second, the Roman Catholic definition of idolatry used above says "the idolater credits the image he reverences with Divinity or Divine powers." Again after searching all the sources listed above, divine powers is not defined. The best I found was "divine power preserved Christ's body from corruption." (CCC 627) and "Divine power is inexhaustible" (Catholic Encyclopedia, Omnipotence). Roman Catholicism does not attribute divinity to Mary (at least, not yet), but it does attribute divine powers to her: atonement of sin12; divine access to God13, delivering our souls from death14; her intercession brings us salvation15, etc.
Such magnificent attributes of Mary are found nowhere in Scripture. Let me say this again, none of these things are found in God's inspired word. They are, however, said to be found in the Roman Catholic Church's Sacred Tradition. I cannot help considering what God said through the Apostle Paul about not exceeding what is written in the word of God.
"Now these things, brethren, I have figuratively applied to myself and Apollos for your sakes, that in us you might learn not to exceed what is written, in order that no one of you might become arrogant in behalf of one against the other." (1 Cor. 4:6).16
So, the Roman Catholic Church is commanded by Scripture to not exceed what is written in God's word. Has it done that? Has it exceeded the limits of Scripture? Yes, it has.
In the Bible, NASB, the word idol (and its cognates idols and idolatry) occurs 175 times in 162 verses. I read every one of the 175 verses, and not once did I find a reference to the idolators actually considering the idols in their hands and/or to which they bowed as actually being alive because that is what it would mean to claim that they were divine. Remember, Catholicism says, "the idolater credits the image he reverences with Divinity or Divine powers."17. But this is wrong. You see, the Catholic Church is stacking the definitional deck in its favor so as to separate itself from the biblical teaching of idolatry and maintain its practice of bowing down before various statues and praying to saints.
But saying the Roman Catholic Church is practicing idolatry doesn't prove anything. So, let's turn to God's word and take a look at the examples of idolatry and then see if the Roman Catholic Church practices the same thing concerning Mary.
For the table showing how the Catholic church practices idolatry and Scriptural references -
https://carm.org/roman-catholicism-mary-idolatry