Catholicism and Christianity are as different as chalk and cheese, but the key difference, on which every other difference hangs, is authority. Both in the authority we claim, and the authority to which we appeal, the two faiths are fundamentally different. As Christians, we do not claim any authority of our own. We believe ourselves to be stewards of a message we cannot change, nor even withhold, but can only declare. Our authority to do this comes from God, as we find it in His word. This word tells us "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). Furthermore, it tells us that not only can the "simple" understand it, but that it gives understanding to the simple, making them wiser even than their teachers (Psalm 119).
The Apostles themselves did not consider themselves as great as Scripture. They submitted themselves to its tests, and commanded the Christians under their care to test them by its standards. Paul considered himself and his fellow Apostles, to whom he was very much an equal (2 Corinthians 11:5; 12:11), to be "stewards of the mysteries of God" (1 Corinthians 4:1). Moreover, he said stewards must be found faithful (1 Corinthians 4:2), which is in step with James' warning that teachers will be judged more strictly (James 3:1). To Paul, the message had more authority than the messenger, because the God from whom the message came had more authority than the messenger. Thus, He declared that if he or his fellow Apostles, or even an angel from Heaven, should preach a different gospel, the Church should consider them accursed (Galatians 1:8). This Gospel he preached, he was confident would hold up to the scrutiny of Scripture, and so in Acts 17:10-12, he allowed both the Jews and the Greeks to search the Scriptures daily to see whether he was telling the truth. For this, Luke calls them "noble minded".
Of particular note is that the Apostles themselves could, and occasionally did err. In Galatians 2:11-16, for example, Paul rebukes, of all people, Peter, "...when I saw that they were not straightforward about the truth of the gospel...", and so "...when Peter had come to Antioch, I withstood him to his face, because he was to be blamed..." (Emphasis mine). An Apostle, and one of particular note at that, was worthy of blame because he, out of fear, was not straightforward about the Gospel.
It is quite clear, then, that in Christianity, the only word that ultimately matters is God's, and this we find in His word. Our authority is God, and our authority comes from God.
Catholicism takes a very different approach, both claiming higher authority than it has any business claiming, and constantly appealing to the wrong sources to prove it. To begin with, there is some common ground. Christians and Catholics recognise the same Apostles, and even both acknowledge that the Bible is inspired by God. The difference is in both how this is put into practice, and even the additions the Catholic Church places on top of it.
To begin with, Catholicism officially teaches that "sacred" tradition is equal to Scripture. As we read in Dei Verbum chapter 2, "...both sacred tradition and Sacred Scripture are to be accepted and venerated with the same sense of loyalty and reverence." (Emphasis mine). This immediately causes conflict, as Scripture teaches its own sufficiency as an authority. There is no authority equal to, or greater than, Scripture.
But it gets worse, because ultimately, as Scripture says, no one can serve two masters (Matthew 6:24; Luke 16:13). Thus, while the Catholic Church teaches the equality of Scripture and "Sacred" tradition, the master Catholics typically end up hating is the Bible.
This is primarily because Catholicism also teaches "...no one, relying on his own skill, shall,--in matters of faith, and of morals pertaining to the edification of Christian doctrine, --wresting the sacred Scripture to his own senses, presume to interpret the said sacred Scripture contrary to that sense which holy mother Church,--whose it is to judge of the true sense and interpretation of the holy Scriptures,--hath held and doth hold; or even contrary to the unanimous consent of the Fathers; even though such interpretations were never (intended) to be at any time published. Contraveners shall be made known by their Ordinaries, and be punished with the penalties by law established." (Council of Trent, Session 4, "Decree Concerning the Edition, and the Use, of the Sacred Books").
We see, then, that where Scripture and tradition disagree, the Bible is always the one set aside. Tradition interprets Scripture, Scripture is never allowed to judge tradition. Otherwise, you can actually be punished by law (though thankfully, the Roman Catholic Church has since lost its power to enforce such penalties).
What's especially ironic about this decree is the claim about the "unanimous consent of the Fathers". To begin with, the problem with the title "Church Father" is they were not the fathers of the Church. According to Jude, the Christian faith was "...once for all delivered to the saints" in the first century. This makes the writings of later men, including the so-called "Fathers", completely irrelevant; you cannot father that which precedes you. Thus, even if you genuinely could point to a single issue in which those bearing the common moniker "Church Father" were unanimous, it would be utterly pointless to do so. As I like to say, "why cite a lawyer when you can cite the King?"
But what's more is that there are few things on which they were truly unanimous. They frequently contradicted each other, and even themselves. Being men, they were by no means perfect. They sinned, they erred, and they even grew. Augustine in particular wrote a whole work entitled "The Retractions", in which he reviews and corrects his previous works. This means it is actually possible to find Church "Fathers" espousing views they did not ultimately hold at the end of their lives. For Augustine, this, ironically, includes the view that Peter was the rock referred to in Matthew 16:18:
"In a passage in this book, I said about the Apostle Peter: ‘On him as on a rock the Church was built’...But I know that very frequently at a later time, I so explained what the Lord said: ‘Thou art Peter, and upon this rock I will build my Church,’ that it be understood as built upon Him whom Peter confessed saying: ‘Thou art the Christ, the Son of the living God,’ and so Peter, called after this rock, represented the person of the Church which is built upon this rock, and has received ‘the keys of the kingdom of heaven.’ For, ‘Thou art Peter’ and not ‘Thou art the rock’ was said to him. But ‘the rock was Christ,’ in confessing whom, as also the whole Church confesses, Simon was called Peter. But let the reader decide which of these two opinions is the more probable."
This is neither the first time, nor the only time, a Church "Father" sounded more "Protestant" than Catholic (though of course, in the earliest days of the Church, there were no Protestants, because there was no Catholic Church to protest). In fact, whereas Catholics are quick to memorise and regurgitate anything that sounds remotely Catholic in the writings of the "Fathers", they are quick to point out that they were not infallible whenever it turns out the same "Father" said something very anti-Catholic, even if it is in the same book! In other words, in practice, both Catholics and Christians see the "Fathers" in a similar way. To Christians, there is some benefit in learning their teachings. They had insight into their particular times, and some of them even had direct contact with the Apostles. Nevertheless, they were not infallible, and absolutely not unanimous. Much less unanimously Catholic.
Nevertheless, due to their proximity to the Apostles, their word does hold more emotional weight than it should. The "Fathers", people reason, were in a better place to know certain things than we are. It is seen as arrogant to disagree with them (even though agreeing with one of them naturally leads to disagreeing with another), simply because they came before us. The earliest even had direct contact with the Apostles, up to and including being direct students thereof.
But notice the contradiction here. On the one hand, we are being told to unquestioningly believe the direct students of the Apostles. On the other hand, this is used as an excuse not to believe... the very Apostles to whose authority is being appealed! If we have to believe, say, Polycarp, since he was the direct student of John, does it not make sense to just believe John?
In fact, let's take a moment to speak about John. See, John, being a direct disciple of Jesus, and being inspired by God Himself to write Holy Scripture, gives us a far greater insight into the Christian faith than all of the Church "Fathers" combined. God is greater than man, and teacher is greater than student. It's simple logic. Well, John actually gives us a little lesson on how to interpret Scripture.
In John 21:23-24, we read "Then this saying went out among the brethren that this disciple would not die. Yet Jesus did not say to him that he would not die, but, “If I will that he remain till I come, what is that to you?” This is the disciple who testifies of these things, and wrote these things; and we know that his testimony is true." This shows us, first of all, not to impose on the text what the text does not impose on us. This is a lesson that all Catholics could definitely stand to learn.
See, Catholics constantly impose things on the text that are simply not there. They read the Papacy into Matthew 16:18, but there is no Papacy in the Bible (nor was there for the first few centuries). They take John 19:26-27 to mean Mary is the mother of the whole Church, though it does not say that; it only says Jesus gave Mary to John as his mother. They read John 21:25, and use that to suggest the Bible is insufficient, and there are other oral traditions of the Apostles preserved only by the Catholic Church.
But more to the point, note the response to Jesus' words. The reason John clarifies "Yet Jesus did not say to him that he would not die..." is because even among the brethren, there was a false tradition spreading that John, who had not yet died, would not die.
Now, it does not specify exactly who "the brethren" were. They have to be Christians, or it's unlikely John would refer to them as "the brethren", but we can grant they were not necessarily the Apostles. This means falsehoods actually spread, and rapidly enough to be noted in God's word, before the Apostles died. In fact, what you will find is that a lot of the New Testament was written as a direct rebuke of false teachings which had deceived even the believers!
So, let's suppose we found a lost letter of Polycarp, John's disciple, in which the saying "John will remain until Christ returns" is found. No matter who said John would never die, no matter where they got such an idea, such a saying should never have been believed, because it did not come from God. John himself could not have said "I will never die", and when the Scriptures he wrote said "but Jesus never said that", that is what should have been believed.
The same principle applies to anything that might be found throughout history. Scripture takes precedence over Justin Martyr's anti-Semitism, Jerome's claim that having too much sex with your own wife is adultery, or Augustine's belief that women are useless to men apart from the purpose of childbearing. Find me a Church "Father" that believes anything, I will judge him not by his proximity to the Apostles, but by how well his beliefs match the teachings of Scripture, and ultimately, throughout history, there have been teachings from the mouths of the Apostles themselves which have fallen short.
But of course, that doesn't take away legitimate authority. Moses, for example, sinned so grievously that he was denied entry to the promised land, dying on top of a mountain that overlooked it. Yet, he wielded such authority that even Jesus appealed to his writings. So how do we reconcile this? Put simply, by taking the opposite approach to the Catholic Church. In Catholicism, tradition interprets Scripture. In Christianity, Scripture judges tradition.
Despite all their protestations, Catholic tradition cannot withstand Scripture. That's why they need to claim so much authority in the first place. The Marian dogmas run contrary to Scripture. Purgatory runs contrary to Scripture. The 7 Sacraments, while some of them bear some resemblance to the teachings of Scripture, run contrary to Scripture. The very Gospel, as it is found in Scripture, is anathematised by the Roman Catholic Church. Strangely enough, it is even possible to follow history all the way back to the true origins of the majority of Catholic doctrines. We find, for example, that Catholicism did not always teach Transubstantiation, and that the modern version originated in a dispute between two monks in the 9th century. As it stands, the historical evolution of the Catholic Church is so well documented, their scholars even invented a whole concept to explain why the early Church in no way resembles the modern Catholic Church.
But let's suppose that wasn't true. Let's suppose you could genuinely trace every Catholic tradition all the way back 2,000 years. Let's even imagine that we could prove the book of Romans was immediately handed to a man calling himself the Pope, claiming to be the immediate successor of Peter, with the capability to make infallible declarations about doctrine, and being preferable to the other Apostles individually or collectively. Our only response to such a discovery would be to treat this band of heretics exactly as we treat the Gnostics, or the Judaisers, or the hundreds of other false teachers who carried Satan's lies across the world before the ink of the New Testament Scriptures was even dry.
We see, then, that Catholicism is rotten to its very foundations. While it ironically claims to be the Church Jesus founded on "the rock", i.e. Peter, Peter himself would take one look at this Church and say "may this abomination sink below the sands".
And in time, it will, for Christ warns us “Therefore whoever hears these sayings of Mine, and does them, I will liken him to a wise man who built his house on the rock: and the rain descended, the floods came, and the winds blew and beat on that house; and it did not fall, for it was founded on the rock. “But everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand: and the rain descended, the floods came, and the winds blew and beat on that house; and it fell. And great was its fall.” (Matthew 7:24-27).
Catholics should therefore be advised to abandon the Church that says "our will, not God's" (or, as one Bishop said to William Tyndale, "It’s better to be without God’s laws than the pope’s."), and build your house upon the true rock, that is, the words of Jesus. All other ground is guaranteed to lead to your eternal ruin.