Some of the reasons why protestants are against Roman Catholic teachings:
God several times throughout Scripture says He will not share His glory,
while Catholics glorify, venerate, and adore Mary.
Jesus did not once in Scripture address Mary as His mother,
but Catholics call her the mother of humanity, and as if that
were not enough, the second Eve, and the queen of heaven.
Jesus never elevated Mary above others, but Catholics feel a need
to not just elevate Mary above others, but they put her on a par with
Jesus in terms of His sinlessness, (do you really think that she needed
to be sinless so that Jesus Christ could be sinless?).
Jesus said that anyone who did the will of God was His mother, His brother, His sister.
They are forced by so-called infallible papal decree to accept the Marian
dogmas of her bodily assumption and immaculate conception, neither of
which have any basis whatsoever in Scripture.
Jesus told us to pray to our Father in heaven, yet they pray to Mary,
because their popes tell them to despite what Jesus explicitly instructed.
Scripture clearly states that there is one intercessor between God
and man, yet their popes, bishops, priests, parishioners etc, will
tell you to pray to Mary, and other dead people, for intercession.
They fault those who do as Jesus suggested, while they
disobey Him to follow someone else instead.
We see nothing in Scripture to endorse her bodily assumption,
her perpetual virginity (Jesus had siblings after all, two of
whom have books in the Bible!), or her immaculate conception.
We see no need to pray to her, or any ability on her part, as dead
as she is believed to be, to intercede on our behalf against the
explicit words of Jesus in Scripture.
Within Catholicism, there is a drive to define a new Marian dogma
in which Catholics, as a matter of faith, would be obliged to accept:
1) Mary participates in redemption with Jesus Christ;
2) grace is granted by Jesus only through the intercession of Mary;
and 3) all prayers from the faithful must flow through Mary, who
brings them to the attention of her Son.
Scripture also tells us not to call anyone Father but God,
while they call many men "father" and blindly follow them.
They believe that a fallible man is the Rock that Jesus is building
His Church on, not a confession of faith divinely inspired by the
Holy Spirit of God, despite a plethora of Scriptures that attest
to the fact that God is the sole Rock of our salvation.
They teach purgatory, and many other things neither the apostles
nor Jesus ever taught. For instance, they claim priests must be celibate,
when they were not in the early church.
Another example would be infant baptism, nowhere promoted in Scripture.
They burned people at the stake for daring to read the Bible, which is what
Scripture tells us to do. Despite all this, and more, the Roman Catholic
Church considers herself the only valid expression of the community of God.
