The number one tradition of Protestantism is private interpretation of the Bible (Sola Scriptura). According to Luther and others: under the guidance of the Holy Spirit any Protestant can inerrantly read and properly interpret the Bible. Now there is no mention of this tradition anywhere in the Bible. In fact, in the Bible, the opposite is taught. For example... (2Peter 1:30) "Know this first of all, that there is no prophecy of scripture that is a matter of personal interpretation" and in 2 Peter 3:16, referring to the letters of Paul, Peter says "...in them there are some things hard to understand that the ignorant and unstable distort to their own destruction just as they do to the other scriptures.".
Certainly, reading the Bible is a good thing and beneficial to our spirits. And much of the Bible is easy to understand. But there are many places in the Bible that are ambiguous and hard to understand. If the Holy spirit guided all people in interpreting the Bible there would be no disagreements in interpretations. Now there are thousands of different protestant denominations that are the result of thousands of different interpretations of the Bible. Logically, this would mean that private interpretation is not guided by the Holy Spirit, at least for some.
In the Bible there are seeming contradictions. For example" 1 John 3:9 says " No one born of God can sin; for God's nature abides in him and he cannot sin because he is born of God. But 1 John 1:8 says "If we say we have no sin, we deceive ourselves and the truth is not in us". How is this to be explained ? Protestants sometimes add words to the Bible such as "No one born of God can continue to sin;...". Others just ignore it.
The Catholic Church explains it this way. There are major sins that break your relationship with God; like adultery and murder. This is the type of sin addressed by 1 John 3:9 explaining that we cannot break our relationship with God if we are born of God...if we keep Him carefully in our heart. Then there are lesser sins that weaken the relationship but don't break it. 1 John 1:8 deals with such smaller sins. We all commit many small sins each day because we are fallen and imperfect. But such small sins don't break but they do weaken our relationship with Jesus: such as saying an unkind word or arguing with a sibling. This is also dealt with in 1 John 5:16-17. "If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and he will give him life. This is only for those whose sin is not deadly. there is such a thing as deadly sin about which I do not say you should pray. All wrong doing is sin but there is sin that is not deadly. So this explains the seeming contradiction.
The word Protestant means one who protests. What does he protest? He protests the authority of the Catholic Church. Protestants are all united in that point. That started with Martin Luther. But protestants are not united in other teachings of Christ. One denomination teaches that Christ was not God but His representative. Another teaches that God was created a man like us. Protestants differ on infant baptism and whether the Lord's Supper is only symbolic. Authority is the only answer to these different interpretations, but Protestants recognize no authority.
Except there is one area where the authority of the Catholic Church is recognized by Protestants; where the Bible came from. Protestants credit the Catholic Church for selecting the books of the New testament although some say it was obvious which books belonged. But that's not true. In the early centuries before the Bible was complete there was much argument. After Constantine made Catholicism legal in 325, it was easier for the bishops to gather to discuss what books were needed. At the time there were perhaps 20 Gospels and 80 epistles. Mathew , Mark, Luke, and John were recognized as inspired gospels in the second century but so were the Shepherd of Hermas, the Diatessaron , the Gospel of Thomas, and the Gospel of Peter among several others. The writings of the early Church Fathers record these disagreements. They are a matter of public record. It took 3 councils to finalize the books of the New Testament. Around 387 A.D. the Pope approved the complete Bible although books such as Hebrews and Revelation were still disputed by some. The Council of Carthage in 397 reaffirmed the pope's decision of 387.
Since the printing press wasn't invented until 1440, few people before then had a Bible. Even the rich might have only the 4 gospels. A Bible took around a year to copy by hand using expensive parchment, a quill pen, and great care. Until 1440 few people could read or write so the Bible was read to people in church on Sunday. The idea of Sola Scriptura wasn't even possible until 15 centuries after Christ.