Sometimes mistakes are made through being careless, at other times there can be a deliberate attempt to tamper with goods and not to declare that information when the product is sold. Either way there can be a difference between the claims of the familiar label and the reality of the product itself. When that happens, being wise consumers, we don't take deep reassurance from the label that all things must be well because the wording is right. Instead we dispose of the faulty product, and rightly so. It is, after all, not what it claims to be. And not necessarily having all the knowledge or expertise that we need we are grateful for those who monitor and regulate these matters (like the Food Standards Agency who recently pulled some Welsh lamb packed with veterinary drugs off the supermarket shelves).
Well if this is true in everyday mundane matters like shopping and eating how much more so it it true in matters of doctrine. Our assumption is often to treat claims at face value and to gain reassurance from the use of familiar biblical and theological phrases. But heresies and false doctrines involve the preservation of orthodox words and phrases whilst at the same time replacing the intended meaning of those words. 2 Corinthians 11 is a case study in this kind of deception.
This is what makes error plausible and dangerous. It is able to make inroads because the labels it carries all appear perfectly normal. At this point the intellectual and moral dimensions of error are both involved. It is one thing to sincerely misrespresent the truth through a lack of understanding, it is quite another to misrepresent the truth by this misuse of language.
Two obvious ways in which this occurs is in the use of biblical terms (e.g. God, Jesus, Spirit, gospel, hell etc.) and the use of more theological and confessional language (e.g. Trinity, substitutionary atonement, infallibility, covenant of grace). There is of course an overlap where biblical terms have been understood within a certain theological tradition where their meaning has been well established (justification, election). I have no strong assurance that someone suspected of theological mischief is orthodox just because they are able to role out phrases like "I believe that Christ died for our sins" or that we are "justified by faith alone." It is what they mean by those words that matters.
Gresham Machen was conscious of this issue during the Presbyterian conflicts of the 1920s and 30s:
Traditional terminology is constantly being used in a double sense. Plain people in the church are being told, for example, that this preacher or that believes that Jesus is God. They go away impressed; the preacher, they say, believes in the deity of Christ; what more could be desired?Gresham Machen, God Transcendent, p. 44-5
What is not being told them is that the word 'God' is being used in a pantheising or Ritschlian sense, so that the assertion, 'Jesus is God,' is not the most Christian, but the least Christian thing that the modernist preacher says. The modernist preacher affirms the deity of Jesus not because he thinks high of Jesus but because he thinks desperately low of God.
Formerly when men had brought to their attention perfectly plain documents like the Apostles' Creed or the Westminster Confession or the New Testament, they either accepted them or else they denied them. Now they no longer deny, but merely 'interpret.'