Bart Ehrman is the head of the Religious Studies department at the University of North Carolina. In his book Lost Christianities (read my review), he talks about the many different forms early Christianity assumed. Ultimately, he argues, one form (not without its own sub-sects and theological disagreements) emerged as the dominant form. He refers to it as the "proto-orthodox," and rightly states that all forms of Christianity today are its descendants.
I know that many, many protestants would reject this notion. They would claim that they are simply a resurgence of first century Christianity, but their declaration is shot out of the water when they have to answer: which first century Christianity are they manifesting?
Now, while I firmly agree that what became known as the Roman Catholic Church is the common ancestor of all modern forms of Christianity (even those who display many characteristics of earlier heresies), I also maintain that the RCC's contention that they have never changed doctrine is not true (I am NOT calling anyone a liar. I am merely stating that this particular claim is not true and has been promoted by well-meaning but misinformed people, not dishonest, or those who choose to ignore certain evidence).
What was orthodox at one time became heretical in another, and vice-versa. I have come to see that all forms of Christianity today (not the magazine... specifically) have elements that would have been considered heresy in an earlier age... even the RCC.
I was asked about the two most glaring examples of this last night. First, most Christians adhere to a form of gnosticism (the belief that it is special knowledge that brings salvation). If we are saved by grace, and we avail ourselves to that grace through faith (Eph 2:8-9), and the only faith we are capable of having must be given to us from God (Rom 12:3, 1 John 5:4, 2 Peter 1:1) then we require a special, supernatural, not available to all, knowledge to achieve salvation. Plus, the belief that only those who have been specifically empowered by the Holy Spirit can properly interpret scripture is another form of this idea of necessary γνωσις (Greek for knowledge) to be a full Christian. Unfortunately, there are over 2 billion Christians in the world today, each claiming their faith, thus claiming the indwelling of the Holy Spirit, yet biblical interpretation is far from uniform among them.
The second was the early heresy of dualism (Manichaeism... later associated with the supposed Cathars) and its resurgence in modern protestantism. Dualism espouses the belief in two distinct "powers" at work in the world: an ultimate good versus an ultimate evil. Here, good and evil exist in and of themselves. Usually they are personified in God as the supreme good and Satan (or perhaps Osama bin Laden) as the supreme evil, but they are dealt with as if they exist apart from these personifications. Dualism would state that these are equally and opposing forces at work. Now, most Christians would deny that aspect and say that God is more powerful than Satan... good is more powerful than evil. But to assert that these are actual forces at work is to call into question the ultimate good itself. If it is more powerful, why hasn't evil been eradicated? Furthermore, if everything originated from God, how did evil come into existence? Did the entirely benevolent God create evil, and, if so, could God have created a world without evil, and, if so, why didn't He/She? The most glaring illustration of this pervasive, semi-dualistic thinking occurred during the presidential candidate forum at Saddleback. Both candidates were asked if evils exists and what was the proper way to deal with it. Notice the second half of the question already assumes an affirmative to the first part.
How to avoid these? We need to stop claiming a direct line to God. No matter how "sure" we are of our relationship to God and the separation of those we classify as not belonging to our group, we must not assume that communication with God is a dispensation to speak FOR God. God and God alone will do the eternal dividing (I certainly hope there is not one in the line for separation but all are destined for eternity with God and each other). God is the author of all truth and we are simply guessing (I will grant that there is a strong element of educated guessing, but we must never claim to have a divinely revealed truth imparted only to us... what if we're wrong?).
As I've said before, to avoid dualism we MUST stop speaking of evil as a noun! If God is sovereign, completely good, all-loving, all-powerful, then He/She (yes, there appears to be female imagery in the bible for God) would not have created an entity diametrically opposed to God's own nature. Evil is an adjective used to describe actions born from humanity's separation from the ultimate good. In a sense, just like cold does not exist but is the absence of heat, evil is the adjective used to describe people, places, and events that choose to exist outside of relationship with the creator God.
The things to remember about heresy are: that heretics always thought (and still think) they are the orthodox, orthodox beliefs and statements are born through confrontations with heresy, and we are all subject to accusations of heresy in some parts of our theologies. I would go so far as to say there has never been someone (other than Jesus) who was not guilty of holding a heretical belief or engaging in a heretical practice at some point.