In the first chapter, Kruger frames the direction of the book.
If "heresy" (divergent thinking) was the order of the day in the first and second century, and it wasn't until Rome had enough power to vote orthodoxy into place, heresy came first - and was the norm. The idea that there was and "orthodoxy" was heretical (outside of common thinking)
However:
If the writers of New Testament Scripture were unified in doctrine (although not necessarily practice), then there was an "orthodoxy" ("conforming to established doctrine especially in religion" - per Merriam-Webster) before the word "orthodoxy" was used.
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My thinking is that if God, through the Holy Spirit, inspired the writers, He would not have inspired them to say conflicting things. (1 Corinthians 14:33 - For God is not a God of confusion but of peace.) There is no conflicting doctrinal statements in Scripture.
Yes, there was divergent thinking in the early church. Paul addressed it.
I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. Gal 1:6-7
So there were doctrinal differences, but it was not a good thing.
What Walter Bauer misses is the men who codified "orthodoxy" He treats the topic as if they all just got together one day and decided to vote on what they liked best, and "orthodoxy" is no more correct (or incorrect) than the "different gospel."
In this case, "heresy" became heresy because of orthodoxy.
But...if what happened was that false teaching was becoming more prevalent and needed to be addressed by church leadership as a whole, they would have gathered together in prayer and study, in order to determine from Scripture what "orthodoxy" was. They weren't looking for what was most popular, they were looking for what was most true. Orthodoxy was codified in response to heresy - but it was present from the start.
In this case, "orthodoxy" came before heresy.