Because the Protestant reformers rejected the teachings of the Magisterium, they had to come up with an alternative interpreter. They decided that the rightful interpreter of Scripture was each individual Christian, who would interpret the Bible according to the light of the Holy Spirit. The problem with this is that the whole idea of private interpretation of the Scriptures is unworkable, unhistorical, and unbiblical.
It must be said that the Catholic Church teaches the right to private judgment to some degree. Catholics are free to interpret the Bible for themselves (theologians have been doing this for centuries), so long as their interpretations are not in contradiction to an infallible or definitive teaching of the Catholic Church. What the Church condemns, however, is the interpretation of the Bible in a way that is at odds with Church teaching.
The problem with private judgment is that the interpretation of Scripture comes down to our own fallible understanding. The Bible is never the final authority. The person that interprets the Bible becomes the final authority. Since the individual interpreter is fallible, the final authority is fallible. The interpreter of Scripture, therefore, has no way to know whether or not his interpretation is correct. An evangelical has no way to know if he or a Methodist is correct about baptismal regeneration, or whether he or an Anglican is right about the Real Presence of Christ in the Eucharist, or whether he or a Lutheran is right about the possibility of losing one's salvation. Who is to settle these disagreements? According to Protestants, the obscure passages can be interpreted in the light of clear passages. The problem is, some "obscure" passages have no "clear" passages in which to aid in their interpretation. And then we still have the dilemma of multiple interpretations. Baptists use this method and conclude that baptism is merely symbolic. Lutherans use this same method and conclude that baptism regenerates. It becomes necessary, therefore, to have an infallible interpreter to decide, especially since the Bible is not as clear as Protestants think–it can be a difficult book to interpret: "Our beloved brother Paul wrote to you according to the wisdom given him, speaking . . . as he does in all his letters. There are some things in them hard to understand, which the unlearned and unstable twist to their own destruction, as they do the other scriptures" (2 Peter 3:15-16).
Paul writes: "The time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own likings" (2 Tim. 4:3). When Protestants church-hop, they are guilty of this very thing. The problem is, God wants us to know the truth, and he provided the means to do it, so that we would not be confused as to what the truth was: "And his gifts were that some should be apostles, some prophets, some evangelists, some pastors and teachers . . . so that we may no longer be children, tossed to and fro and carried about by every wind of doctrine" (Eph. 4:11-14).
Then there are many practical problems with private judgment. In the 1500's, the idea of private judgment was actually workable. However, in an early era, this would have been impossible. First of all, until the invention of the printing press, Bibles had to be hand-copied. Therefore, until 1440, few Bibles existed, and Christians had limited access to the Scriptures, making it virtually impossible to read it through, compare passages, and come up with interpretations. The best they could do was piece things together based on the excerpts they heard in church. This, of course, is a dangerous practice. Second, until recently, most people were illiterate. There is no way they could have interpreted Scripture without being able to read and flip back and forth between passages. Having someone read it to them would have been an effort in futility. Third, in order to interpret the Bible properly, Christians would have needed commentaries, a Greek/English dictionary, and other support materials, which were unavailable at the time. Even if they were available, remember–most people could not read. No Protestant would dream of formulating his own interpretations without such materials.
Another problem with private judgment is that there is no way for the individual to determine orthodoxy. The best they can do is tally up the opinions of theologians or pastors on each issue, but none of them are infallible, and "majority rules" doesn't necessarily work in Christianity (otherwise the denial of the Trinity by the Arians, who made up the majority of Christians in the fourth century, would have been true)! There is no way for the Protestant to know whose private interpretation is correct. No matter what the Protestant believes, there is sure to be a Protestant in the same town who believes something different. They both go by the Bible alone, and they both claim the other is blinded by their denominational positions. Who is correct? There is nothing to decide between them except for someone's fallible private judgment, or what he thinks the Bible means.
The Reformers taught the right to private judgment, but limited that right to themselves once they began to come to different conclusions on what the true interpretation of the Bible really was (much the way that Protestant pastors do today). Luther called Zwingli "damned" and "out of the Church" for his denial of the Real Presence of Christ in the Eucharist. Melancthon recommended capital punishment for those who rejected the Lutheran teaching on regenerative infant baptism. Calvinist Geneva put the Anabaptists to death. Once again, is there any way to decide whom among the early Protestants were correct, and whom were in error? The truth is, "No prophecy of scripture is a matter of private interpretation, because no prophecy ever came by the impulse of man, but men moved by the Holy Spirit spoke from God. But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them" (2 Peter 1:20-2:2). The "men moved by the Holy Spirit" are the infallible Pope, and the infallible college of bishops when they are united in their public teaching. It is to the Apostles (and by extension their successors) to whom Jesus "opened their minds to the understanding of the Scriptures" (Luke 24:45). The "private interpretation" comes from those who are not authorized to teach, and reduce the meaning of the Scriptures to their own private opinions. When Philip said, "Can you understand [the Scriptures]", the Ethiopian eunuch responded by saying, "How can I . . . unless someone guides me" (Acts 8:30-31). Just as us, the eunuch needed the authorized teachers to help him interpret the Scriptures according to Apostolic Tradition. The eunuch did not yet have the Holy Spirit, but belief and baptism does not come with the ability to correctly interpret Scripture, as Protestants have demonstrated. The Holy Spirit cannot be responsible for the diverse interpretations which has led to 30 000 denominations, because the Holy Spirit is not the "author of confusion" (1 Cor. 14:33). We must "test the spirits" (1 John 4:1). How are the spirits to be tested? "He who knows God hears us [the authorized teachers]. By this we know the spirit of truth and the spirit of error" (1 John 4:6). Why does each Protestant "re-invent the wheel"–deos he really think he is the only one who can find the truth?