When I was much younger, I naively thought that Catholicism was just another "form" or denomination of Christianity with a lot more imagery, stained glass, and funny traditions. As it turns out, there are plenty of people who actually think something like that, so let's look at just a few of the beliefs and practices of Catholicism that are in direct contradiction to the Bible.
Any religion that practices idol worship is far removed from Christianity, and the pagan religion of Catholicism is full of this.
Additionally, the traditional Catholic Catechism (statement of doctrine) intentionally butchers the 10 Commandments by combining the first two, and splitting up the 10th! Their first two commandments leave out the very important part in Exodus 20 that says, "Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me." Catholicism very specifically and unequivocally makes graven images and bows down to them in worship.
Praying to Mary and "Saints"
The Bible makes it clear that we are to pray to God, and Him alone. There is no scriptural basis whatsoever to pray to Mary or any other person in the Bible or anyone throughout history. Mary was a sinner like the rest of us (Romans 3:23). It's blasphemous to pray to a sinner when our intercessor Jesus Christ is more than capable of handling our prayers and needs. Like idolatry, this is worship of mere people rather than giving God the glory.
Confession to a Man for Forgiveness
1 John 1:9 reads, "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness." Only God can forgive sins, as even the unbelieving Pharisees acknowledged (Mark 2:7). A sinner (priest) cannot forgive the sins of a fellow sinner--we need to go to God--and because of Jesus Christ we can go directly to Him in prayer. In fact, the Bible is clear that there shouldn't even BE priests because Christ is our High Priest who lives forever! There is no biblical basis for confession of sins to a priest.
The Pope is Building a Religion with Muslims and Jews
If you still need more proof that Catholicism is NOT Christianity, the Pope is currently building a religious center with Muslims and Jews for the one-world false religion that will be headed for a time by the coming AntiChrist. Additionally, the "Pope" is not a biblical office, and comes from the word for "Father" which the Bible directly forbids calling anyone as a title of authority (not biological father of course) because God above is our Father (Matthew 23:8-10).
These few things just barely scratch the surface of why Catholicism is a non-Christian religion, and in many places it is directly anti-Christian. We must recognize the authority of the Bible and submit to it.