Error and heresy are not always immediately obvious. Some problems are indeed glaring, and ought to require only a functioning pair of eyes and ears in order to determine the problem. God says that something is black; a man says it is white. QED.
At other times, however, error and heresy masquerade under the appearance of—or perhaps more insidiously alongside of—an apparent orthodoxy. A man seems so reliable and gifted. Perhaps he has a genuine gift of oratory; he speaks and writes and preaches with acknowledged clarity and potency in so many ways. He develops a reputation. People want to know what he says. In so many respects he seems spot on. He seems so effective, perhaps even fruitful.
And then you begin to listen to the gaps.