Defining Heresy and Orthodoxy

Inadequate Approaches
It is tempting to say that whatever doctrine is biblical is orthodox and whatever doctrine is not biblical is not orthodox. But this is too simplistic. For example, assuming that only one of the several views (there are at least four) on the Rapture is biblical, it does not follow that the views that are not biblical are therefore heretical. There are some doctrines which, while not in agreement with the Bible, are not so wide of the mark that they must be regarded as heretical.
Another approach that has been taken is to measure doctrines by the doctrinal confessions of some particular denomination. This is fine so long as what is being determined is not orthodoxy but confessional fidelity. That is, if someone wishes to be an ordained minister of a particular denomination, that denomination is within its rights to ask that such a person agree with its doctrines. If someone does not (e.g., if someone disagrees with the denomination's position on speaking in tongues or predestination), then that person should not expect to be ordained in such a denomination. Given the present diversity of denominations, this should be expected.
On the other hand, it is lamentable that the church has allowed itself to be divided over nonessential issues. Thus, adherence to a denomination's particular distinctives should not necessarily be made the test of Christian orthodoxy. Of course, some of the doctrinal stands taken by a denomination may be basic to orthodoxy (e.g., a confession of the deity of Jesus Christ). In such cases, the denomination's confession and orthodoxy coincide.
What, then, should be the standard of orthodoxy? And how should it be determined? Perhaps most troublesome: Who should determine the standard?
Certainly I do not claim to have any particular authority to determine by what standard orthodoxy shall be judged. I claim no special anointing beyond that which all Christians have (1 John 2:20, 27). I make no claims to apostolic or prophetic authority. I am not even an ordained minister. Who, then, am I to judge who is and is not orthodox? Who am I to call anyone a heretic?
My answer to these questions is twofold. First, I am a Christian, and as such have a responsibility to avoid heresy. I can hardly do so if I do not have some idea as to what heresy is. Second, I am a teacher, called by God to the ministry of teaching my fellow Christians sound doctrine. That gives me no special authority or mantle of divine sanction, and I would not want anyone to assume that whatever I say is true. But it does mean that God has given me a special responsibility, and if I am faithful He will use me to guide other believers into a more complete and accurate understanding of His truth. If I am truly faithful, those who are open to God's truth will know that what I say is true — not because I say it, but simply because I have led them to see what has always been in God's Word, the Bible.
Toward Definitions
What, then, is orthodoxy, and what is heresy? First of all, I wish to point out that the term "orthodoxy" is not in the Bible. That does not mean that the concept itself is unbiblical, but that we cannot read off its meaning from biblical texts.
The words "heresy" and "heretic" are in the Bible, and are used in somewhat varying senses. The Jews called Christianity a "heresy" (Acts 24:14), probably meaning they considered it a sect under God's condemnation. But Paul referred to the various factions among the Corinthian Christians as "heresies," that is, "divisions" (1 Cor. 11:19). Here he seems to regard some of these divisions as distinguishing true believers from false believers, but other divisions as simply unfortunate expressions of sinful disunity among Christians, without suggesting that all who belonged to these different factions were lost. Elsewhere, though, Paul referred to "heresies" or divisions as works of the flesh (Gal. 5:20) and said that a "heretic" — a man causing divisions in the church — is perverted and self-condemned (Tit. 3:10-11). Finally, Peter speaks of destructive "heresies" in the sense of doctrines which deny Christ the Lord (2 Pet. 2:1).
From this survey it is evident that a "heresy" in biblical terminology could be merely an unfortunate division among Christians, but in a stricter sense is a divisive teaching or practice destructive of genuine faith and deserving of condemnation. The looser sense corresponds roughly to our modern denominations, while the stricter sense applies most clearly to groups which reject basic Christian doctrines and set themselves apart from the historic church in its many forms. But a "heresy" in the latter sense can have its start, at least, within the church. Whenever heresies in this strict sense arise, Christians are called to separate themselves from those who persist in holding them.
We may therefore define "heresy" in the strict sense as
a teaching or practice which compels true Christians to divide themselves from those who hold it.
Note the difference here: a "faction" or heresy in the looser sense is an unfortunate division separating Christians from one another, and Christians are called to do whatever they can to overcome these divisions (1 Cor. 1:10). But a heresy in the stricter sense is a division separating Christians from non-Christians (or, at best, from Christians who are persisting in grave error), and Christians are called to draw the line and refuse to have spiritual fellowship with those who cross over it. This is not to say that Christians should not show genuine love, compassion, and personal respect for heretics; too often in church history "heretic" has been a hate-word.
How, then, should we define "orthodox"? We might define it as
whatever teachings and practices are sufficiently faithful to Christian principles that Christians should accept as fellow-Christians those who adhere to them.
To put it simply, whatever religious teachings and practices are not heretical are orthodox, and vice versa.
Notice that we have not said that all members of churches which teach heresy are lost. This is no more true than saying that all who are members of churches which teach orthodoxy are saved. In saying that people are heretics, or that they are following heresy, we are not pronouncing judgment on their eternal souls. We are saying that if they follow those heresies consistently, they will certainly be lost. Conversely, in saying that someone is orthodox we are not saying that they are necessarily true Christians with the assurance of eternal life. We are saying that if they follow orthodox doctrine as the basis of their life (and thus trust in Christ alone for right standing before God) they will be saved.
Aberrational Christianity
It might seem that doctrinal discernment should be a fairly cut-and-dried procedure of determining whether a doctrine is orthodox or heretical. After all, we have defined orthodoxy and heresy in such a way that they cover all possibilities. Either a doctrine is such that those who hold it should be accepted as Christians (in which case it is orthodox), or it is not (in which case it is heretical). This might seem to imply a black-or-white approach in which all doctrine is either completely orthodox or completely heretical.
Although doctrinal discernment would be a lot neater and simpler if this were the case, unfortunately things are more complicated — in at least two distinct ways. First, a single doctrine is never held in isolation from other doctrines, but rather is always part of a system or network of beliefs held by a person or group. And sometimes that system of beliefs includes many doctrines which are orthodox as well as some which are heretical. For example, a religious group might hold that the Bible is the Word of God, that there is only one God, that Jesus was born of a virgin and rose from the dead, and yet deny the deity of Jesus Christ. Such a group's belief system is heretical, even though it contains many true beliefs. Moreover, a group's heretical beliefs generally lead them to misunderstand or misapply even those true beliefs they do confess, since the beliefs tend to be interdependent and thus mutually affect one another. Thus, one of the tasks of doctrinal discernment is to sort out which beliefs in a heretical system are actually heretical, which are not, and how the nonheretical beliefs are misapplied because of the heretical system in which they are held.
The second sort of complication to be noticed is that people often hold conflicting beliefs. Because people are often inconsistent, in some cases they may hold to orthodox beliefs but also hold to beliefs that undermine or contradict their orthodox beliefs. The difficulty presented in such cases is to sort out whether the belief system is basically orthodox or not.
For example, many professing Christian groups today confess belief in one God, but also speak of human beings (usually Christians in particular) as being in some sense "gods." This verbal contradiction may or may not betray a real contradiction in the substance of their beliefs. Making matters even more difficult is the fact that these different groups mean vastly different things by calling believers "gods." In some cases it is evident that they really do not believe in one God at all. In other cases it is clear that they are using the word "gods" of believers in a figurative sense such that their confession of one God is not contradicted at all. In still other cases a real tension exists, and it is difficult to avoid concluding that the group in question holds conflicting views.
In order to accommodate this phenomenon, it is helpful to speak of religious doctrines which undermine or are in tension with a group's orthodox beliefs as aberrational. Holding such aberrational views is a serious problem, and those who do so must be considered as being in serious sin and should be treated accordingly. Specifically, those advocating such errors should not be allowed to teach or minister in the church, and those refusing to keep such aberrant views to themselves should be excommunicated.
The charge that a person or group's beliefs are aberrational is a serious one that cannot be made easily. It is arguable that at one level any incorrect belief is at tension with or undermines orthodox beliefs. By aberrational, however, I am referring only to false beliefs which do serious damage to the integrity of an orthodox confession of faith.
The sum of the matter is that doctrinal discernment is a difficult task -- one which requires sensitivity, a sense of proportion and balance, and a deep understanding of what is essential and what is not. New heresies and aberrations are constantly arising, as well as new insights into biblical truth, and discernment is needed to tell the difference. Thus, the task of doctrinal discernment is an ongoing necessity in the Christian church.
Having shown that doctrinal discernment is necessary, I have yet to say very much at all about how it is to be done.