How can Protestants claim to be guided by the Holy Spirit in contradicting Marian Dogmas?
To remind myself as much as anyone, briefly, the four Marian Dogma's are:-
The Divine Motherhood, Mary can be called "the Mother of God";
Her Perpetual Virginity; her marriage with Joseph was never consummated;
Her Immaculate Conception, Mary was free from original sin;
Her Assumption into Heaven, Mary ascended into Heaven by the power and grace of God.
These teachings the Roman Catholic Church teaches it received from the Holy Spirit, a very normal and natural claim if it is true, but an extremely blasphemous claim if it is not true.
I believe that behind these teachings is another teaching of the Roman Catholic Church, namely, that the Church chose which writings were the Scriptures of God, and that the Church therefore has authority over the doctrines of God and by extension the Church has authority over the Scriptures. The Protestant Churches deny this authority: for us who are Protestant, it is the Scriptures which have authority over the Church; the Church did not choose the Scriptures, but rather the Scriptures are self-authenticating and chose themselves. When believers selected which writings are divine they merely recognised the Scriptures for what they are, they did not have any authority in choosing them.
For the Protestant adding "Christian" doctrines which are not found in Scripture is heresy. The Scriptures are our sole authority in matters relating to our faith. This we gather from the Scripture itself:
All Scripture is given by inspiration of God and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: that the man of God may be complete, thoroughly equipped for every good work (2 Timothy 3:16,17).
From this Scripture we understand that the believer is fully equipped by the Scriptures alone. It is through the Scriptures that the man of God is made "complete" and "thoroughly equipped". This verse of the Bible, tells us very clearly that the man of God, the believer in Christ, is completely, thoroughly equipped for every good work through the Scriptures alone. There is no need for any additional tradition or teaching; in fact such teachings, in that they are a distraction from the Scriptures are dangerous and wrong.
The question might legitimately be asked "Why would any believer want to add anything to God's Word?" About the Word of God the Psalmist says:
the judgements of the LORD are true and righteous altogether. More to be desired are they than gold, yea, than much fine gold: sweeter also than honey and the honeycomb. Moreover by them is thy servant warned, and in keeping them is great reward (Psalm 19:9-11).
The law of thy mouth is better to me than thousands of gold and silver (Psalm 119:72).
There are many similar sentiments expressed throughout Psalm 119.
Our Lord Jesus, in the Scriptures, tells us what is the role of the Holy Spirit:
But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of Truth, which proceeds from the Father, he shall testify of me. (John 15:26)
He shall teach you all things and bring all things to your remembrance, whatsoever I have said unto you (John 14:26).
The Holy Spirit, then, only reminds us of what is said in the Scriptures: it is no longer his intention or role to give us new doctrine, but merely to remind us of what has already been given through the Word of God, the Scriptures.
There are only a few more words to add. In Proverbs we read these words:
Every word of God is pure: he is a shield unto them that put their trust in him. Add thou not unto his words, lest he reprove thee, and thou be found a liar (Proverbs 30:6).
The same sentiment is found at the very end of the Bible, where our Lord Jesus says:
For I testify unto every man that hears the word of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: and if any man take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
I would say the Roman Catholic Church needs to make a public confession of its folly in adding doctrine which cannot be substantiated from the Scriptures.
For more on "The Divine Motherhood of Mary" see:
Why do Protestants not refer to Mary by the title "Mother of God"?
and for more on "Mary's Perpetual Virginity" see:
How does Protestant Church explain Luke 2:48?