In some comments the other day, it was brought up that in some cases, some Protestant doctrines are closer to Catholic doctrines than they are among themselves. This cannot be, and the charts nearby show why. Roman Catholicism and Protestantism have two different "objects of faith." What you have here are the models of how these "objects of faith," and the doctrines themselves, work together. This is to show how they relate. (And these are just to represent concepts of the churches -- they are not intended to be comprehensive).
In the model showing the Churches of the Reformation, salvation is by Christ alone. Christ alone, and Him crucified, is the object of our faith. Explicating this, there is a core of orthodox beliefs, surrounding Scripture, God, Christ, man, sin, redemption, etc. That is, the doctrines explain how Christ effects this salvation. With some small exceptions, these doctrines, especially for the first 100 years or so after the Reformation, virtually all the big and important doctrines were the same among the Protestant churches. Any differences that existed among these churches were to be found not in the core doctrines, but in some of the peripheral ones. (And my list is taken from the order given in many systematic theologies of what is known as "theology proper" -- again, this is not intended to be representative of any one school of thought, but just to be representative of how things worked, in order to illustrate the contrast between how Roman Catholics think of their doctrines, and how Protestants think about doctrines.)
The other chart shows the Roman Catholic view of things. For the Roman Catholic, salvation is "through the Church to Christ." Roman Catholic doctrine is "a seamless garment". It is a whole, an entirety unto itself. There is no extricating any one doctrine from this circle of doctrines. This is what Rome calls "the fullness of the faith" or "the entire deposit of faith." It is also what is believed to be "the faith that was once for all delivered to the saints" (Jude 3). Here is how the CCC describes it:
84 The apostles entrusted the "Sacred deposit" of the faith (the depositum fidei), contained in Sacred Scripture and Tradition, to the whole of the Church. "By adhering to [this heritage] the entire holy people, united to its pastors, remains always faithful to the teaching of the apostles, to the brotherhood, to the breaking of bread and the prayers. So, in maintaining, practicing and professing the faith that has been handed on, there should be a remarkable harmony between the bishops and the faithful."Note that "this Magisterium", while it claims to be "not superior to the Word of God, but is its servant," really has seen the Scriptures over the centuries not for what it is -- the Word of God to be listened to and obeyed. Rather, as Pius XII and others have described it: "theologians must always return to the sources of divine revelation: for it belongs to them to point out how the doctrine of the living Teaching Authority is to be found either explicitly or implicitly in the Scriptures and in Tradition." That is, start with this circle, then use the Scriptures (and Tradition) as the source for proof texts.
85 "The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ." This means that the task of interpretation has been entrusted to the bishops in communion with the successor of Peter, the Bishop of Rome.
86 "Yet this Magisterium is not superior to the Word of God, but is its servant. It teaches only what has been handed on to it. At the divine command and with the help of the Holy Spirit, it listens to this devotedly, guards it with dedication and expounds it faithfully. All that it proposes for belief as being divinely revealed is drawn from this single deposit of faith."
87 Mindful of Christ's words to his apostles: "He who hears you, hears me", the faithful receive with docility the teachings and directives that their pastors give them in different forms.
Scripture is not to be understood as God's Revelation over time to man; for Roman Catholics, what God has revealed is precisely "the circle"; For the Roman Catholic, the object of faith, the "rule of faith" is precisely "the circle," the sum-total of Roman Catholic doctrine today.
Scripture is just one source must be combed through to find texts which might be used, however obliquely ("implicitly"), to support current Roman dogma. What counts for Roman Catholics is this "Entire Deposit of Faith". Even Christ is only one component of this circle, and he is thus part of the background. The "Church Teaching" is in the foreground. "Through the Church to Christ." Yes, Christ's work is essential to the Roman Catholic. But Christ's work is only mediated to you, the individual believer, by the Church's efforts and processes (i.e., through "the Sacraments").
And you, if you are to be a Roman Catholic in a state of grace (which is necessary if, when you die, you are to enter heaven), must accept all at once, all of it, everything that the Roman church teaches, without question, if you are to be a Roman Catholic. This is not "subscription". There are no objections or exceptions. It's all or nothing.
The thing about this system is, the acts of the evil popes are outside of this circle. (This I call "the Alias Smith and Jones defense of the papacy: for all the trains and banks that they robbed, they never taught anyone." That is, their evil acts did not contribute to "the circle"). Abusive priests and the bishops who hide them and give them comfort are outside of this circle.
Roman Catholic theology distinguishes between fides quae, what Aiden Nichols called "the faith of the Church" [which essentially is "what the Roman Church has come to believe over the centuries, and the sum total of what it requires you to believe"], and fides qua, which is the individual's act of having faith.
For the Protestant, there is one object of faith (fides quae): Christ alone. The Protestant acknowledges that there is one God, and there is one mediator between God and men, the man Christ Jesus.... Since then we have a great high priest who has passed through the heavens, Jesus, the Son of God, let us hold fast our confession. For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin. Let us then with confidence draw near to the throne of grace, that we may receive mercy and find grace to help in time of need.
With this foundation in place, then, we "search the Scriptures". As Turretin notes, "Paul appeals to the judgment of believers to prove all things and to hold fast to what is good. John wishes believers to try the spirits whether they are of God. Surely this could not be said if this examination were either impossible or dangerous to them."
In Roman Catholicism, "fides quae" is the entire circle. The object of faith is the entire circle. In Roman Catholicism, there is the entire body of doctrine, "the fullness of the faith" which is what must be accepted. Rome presents its body of doctrine as "a seamless garment," and when you "have faith," that is, when you have "fides qua," it is that entire circle, that entire body of doctrine that you must believe. You must go "through the Church to Christ." You cannot pick and choose among Roman Catholic doctrines on things. You have to swallow the thing whole.
You can't say, "Rome has a pretty good doctrine of 'X'; I'll work toward bringing that knowledge to my church, without bringing the wrong things that Rome teaches into it." It doesn't work that way.
Roman teaching must be accepted in its totality, or rejected in its totality.