Everyone thinks that they are orthodox. No one really thinks that their theology is heretical (unless of course they are seeking to be provocative and attention seeking, but then that is a rhetorical ploy). But there is more to this than simply saying that we all think that our theology is right. Unorthodox theology has every intention of pushing orthodoxy out of its own home as if orthodoxy was the intruder. When it does so it it wages a verbal campaign against true orthodoxy by portraying it as backward, negative, defensive, critical, suspicious, unloving, harsh etc. (in point of fact plenty of this mud, in the 1920s and 30s, was flung in the direction of J. Gresham Machen).
As the following extract shows, open theism presented itself not only as a valid option for evangelicals to hold to, but also as affirming and upholding the doctrine of God's omniscience. The problem with that claim was simply that the open theist doctrine of God's omniscience had never, ever, in two thousand years of church history, ever been an acceptable version of what Christians have meant by omniscience.
"At what point...does an interpreter have a right to say, "I know you SAY you believe in omnipotence and omniscience, and you may even really BELIEVE that you do hold to these attributes, but in fact you do not. You regularly defend positions that directly challenge these terms as they have been understood and used (and isn't meaning in the use?). Even on an etymological rendering, your stated commitments to these divine attributes die the death of a thousand qualifications"?
From The Nature of Evangelicalism, a dialogue between Michael Horton and Roger Olson, available online here