Despite theological protests to the contrary, any additional mediation with God is an affront to the all-sufficient, divinely appointed mediatorship of Jesus Christ: "There is... one mediator between God and men, the man Christ Jesus" -1 Timothy 2:5.
For "For we do not have a high priest who is unable to empathize with our weaknesses, but we have one who has been tempted in every way, just as we are--yet he did not sin. Let us then approach God’s throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need" -Hebrews 4:15-16.
There is no reason to go to Mary or any other saint in heaven with our requests.
This does not mean we cannot ask other living believers on earth to pray to Jesus for us. In fact, the Bible exhorts those of us living on earth presently to pray for each other.
However, understand that we are asking other earthly believers to pray for us and not communing with the dead (e.g. communicating with people who have already physically died which the Bible strictly forbids); we are not asking these living believers on earth to themselves give us grace or mercy (as Catholics do of Mary) but instead asking God for grace and mercy; (3) we are not praying to other earthly believers but asking them to pray for us.
Catholic apologists attempt to make a distinction between Christ as the sole mediator and all believers as intercessors but this does not help their cause (of proving we should pray to saints), because all the passages they employ are about direct intercession in prayer to God, not to other creatures. No biblical passage states or implies that we should pray to the saints. Catholic dogma, which maintains infallibly that we should, places tradition over Scripture thereby proving the fallibility of the magisterium.
Catholic rationalization for praying to the saints is also based on the seemingly plausible argument that because of their position in heaven, they may be better able to intercede. This, though, rejects the ministry of the Spirit, whose task it is to do this on our behalf. Who is better able to intercede for us than another person of the Trinity?
"We do not know what we should pray for as we ought, but the Spirit Himself makes intercession for us with groaning which cannot be uttered" (Romans 8:26)"; through Him [Christ] we... have access by one Spirit to the Father" (Ephesians 2:18). Since beyond our explicit prayers to God, the Holy Spirit intercedes for us perfectly "in accordance with God's will" (Romans 8:27), there is no need to call on anyone else in heaven to do so. Start with 1 John 2. It is wrong to expect any person to be more efficacious with God the Father than God the Son and God the Spirit all of whom are omnescent and immutability so.
This; however, does not equate to your assertion that "anyone who claims to be a Christian, and is praying to Mary instead of the Lord Jesus Christ, is not a Christian but has been deceived by the father of lies!" is factual.
In fact, it's not because the requirements for salvation do not exempt anyone in the world who prays to Mary from receiving salvation.
People around the world are deceived about a great many things including Catholics praying to Mary; however, God doesn't require perfect doctrinal understanding to receive salvation. Instead, the Bible states they receive salvation by accepting the Gospel and being "born again." (see John 3). Afterwards, comes progressive sanctification and the renewing of the mind.
I fully agree that praying to Mary is complete Idolatry! And anyone who claims to
be a Christian, and is praying to Mary instead of the Lord Jesus Christ,
is not a Christian, but has been deceived by the father of lies!