After mentioning them in my last blog post, I just have to find out who the Cathari are. So far, I only have information from Wikipedia.
From what I can see, all of them were nontrinitarian. Some were modalists, believing that God reveals himself three ways and denying the individuality of Father, Son, and Spirit. Some were adoptionist, saying that Christ is more of a demigod than equal to God the Father. Apparently, they based their theology on the Bible, causing the Bible to be forbidden from the ordinary people for fear of uprisings.
As a devout Presbyterian, the church should base all of their beliefs on the Bible. They should take the Bible over the popular vote. However, we are not to form individual beliefs based on choosing Scriptures here or there. That is what makes the Bible so alive: if we are uncomfortable with it, we have to deal with it. This is why God gave us the Church. In Christ's physical absence, the church needs men led by the Holy Spirit to study God's word and to give correct theology, making it available to all the people. There should be many ministers leading the church so that one person does not do what the Cathari did, making an individual interpretation of Scripture unchecked by other wise scholars.
This is what makes the Lutheran Reformation suspect. Luther read the Bible then worked to reform the church. He caused a revolution. Even he acknowledged that people can used the Bible to decide what kind of ale they should drink if left to individual interpretation. The difference: Luther did not want to leave the Church. The Protestants met with councils for centuries to solidify their beliefs based on Scripture so that all church-goers would agree on the same important theology. This is why the Church has creeds and confessions.
The Cathari separated from the Church and formed their own individual beliefs that they can prove by maybe one Bible verse, but they don't have a defined confession or creed, so they have nothing to stand on that Christ actually instituted. The Lutherans did not want to separate. They wanted all of the Church to agree on the Words of Scripture and their interpretation. Some Catholics placed their emphasis on Church interpretation of Scripture. The others wanted to go back to Scripture to correct errors in the church. The latter became the Lutherans and eventually became excommunicated. While I'm on the Lutheran side of the Reformation, I still see that as the one point where the Church fell apart, causing Satan to influence it more because of the division. That may have been the turning point where Satan was released from captivity, if you follow the amillennial perspective.
But now, to remember the point of all this. If some person has a Biblical interpretation that nobody in history accepted by the church has ever had, then they are a heretic, or a Cathari. If someone's Biblical interpretation can be verified by the whole of the Bible and can be attested to through men like Augustine or Aquinas or John Chrysostum, then it is a legitimate interpretation because the Church is a collective body, not a splintered mosaic. The latter is what you should follow and what the Church should heed to stay healthy.