Rather than list out the obvious disagreements of "ever virgin", "mother of God", "queen of heaven", the "immaculate conception", and the aptly named "assumption", why don't we talk about praying to saints in general.
1 Timothy 2:1-7
1 Therefore I exhort first of all that supplications, prayers, intercessions, and giving of thanks be made for all men, 2 for kings and all who are in authority, that we may lead a quiet and peaceable life in all godliness and reverence. 3 For this is good and acceptable in the sight of God our Savior, 4 who desires all men to be saved and to come to the knowledge of the truth. 5 For there is one God and one Mediator between God and men, the Man Christ Jesus, 6 who gave Himself a ransom for all, to be testified in due time, 7 for which I was appointed a preacher and an apostle—I am speaking the truth in Christ[a]and not lying—a teacher of the Gentiles in faith and truth.
The implication of the first boldfaced type is that living people pray for other living people
The implication of the second boldfaced type is that only one person petitions God the Father on behalf of man - Jesus Christ, having earned that right through perfect obedience, even death on the cross.
Matthew 27:51-53
51 Then, behold, the veil of the temple was torn in two from top to bottom; and the earth quaked, and the rocks were split, 52 and the graves were opened; and many bodies of the saints who had fallen asleep were raised; 53 and coming out of the graves after His resurrection, they went into the holy city and appeared to many.
Sleeping saints don't hear or respond.
Acts 9:41
41 Then he gave her his hand and lifted her up; and when he had called the saints and widows, he presented her alive.
When Peter called for the saints, did he call the dead or the living?
Romans 1:7
7 To all who are in Rome, beloved of God, called to be saints
Is Paul writing to the dead or the living?
Romans 8:26,27
6 Likewise the Spirit also helps in our weaknesses. For we do not know what we should pray for as we ought, but the Spirit Himself makes intercession for us[a] with groanings which cannot be uttered. 27 Now He who searches the hearts knows what the mind of the Spirit is, because He makes intercession for the saints according to the will of God.
Who's making intercession for whom?
1Corinthians 1:2
2 To the church of God which is at Corinth, to those who are sanctified in Christ Jesus, called to be saints, with all who in every place call on the name of Jesus Christ our Lord, both theirs and ours:
Is Paul addressing dead saints, or living?
2Corinthians 13:13
13 All the saints greet you.
Are dead people saying, "hello"?
Jude 1:3,4
3 Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints. 4 For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into lewdness and deny the only Lord God[a] and our Lord Jesus Christ.
Is Jude warning about the problems ungodly men can cause to the dead saints or the living?
And so on, and so on, and so on. When the writers of the New Testament use the word "saints" they refer to the living, except in specific instances that are either directly stated by the writer or obvious in the context of the letter (The Revelation, for example). In EVERY instance where prayer or intercession is mentioned relative to saints, it is regarding the living NOT THE DEAD (That includes the Revelation 8... the bulk of Scripture makes it plain that the "prayers of the saints" are from those still living).
With this volume of evidence, how is it that the RCC decides that it's okay to pray to dead people as one would to God?
And I haven't even touched on the idolatry argument!
I find the simple examples to be the most obvious and easy to remain in fellowship when observing, for example, why would any group of believers in Christ lead unbelievers or new believers to a further understanding of God through faith in Christ by taking on the title of "Father"? (Even our Lord and Savior Christ Jesus refused such a title.)
The selective nature of their sainthood has always amazed me. Some never make it because they disagreed with the phony doctrines of the RCC,like Tertullian, and yet others of questionable character have been sainted. And the criterion for getting one's halo: apparent miracles that occur on a certain day assigned to the deceased applicant. Where did that ever come from?