My teacher was an old style facts and dates kind of guy. He taught by writing notes onto a blackboard. We copied them down. There was no questioning, no dissent. Nothing to suggest that the details of history were contested, etc. But presumably people would not want to claim that I was being anything like brainwashed by my history teacher…
My suggestion: legitimate instruction exhibits epistemic sensitivity, or - more loosely - is responsive to reasons and evidence. If the facts and dates of history had been different, then so would the History Teacher's instruction have been. His teaching, though itself apparently 'dogmatic', is embedded in a broader academic system (of textbook writers, etc.) that is broadly reliable and responsive to evidence.
The same is not true, we may suppose, of most religious instructors. Even if Plantinga or other philosophers of religion established the truth of theism, they are too disconnected from most religious instructors to protect the latter against charges of brainwashing. The Sunday School teacher would teach much the same things no matter what the best philosophers discovered. Even if their teachings by some fluke happened to be true, it still fails to be the case that they are teaching it because it is true.
Just as a true belief may fail to constitute knowledge due to the holding and the truth of the belief not being related in the right way, so too a true or justified proposition may fail to be taught legitimately -- and instead constitute brainwashing -- because the act of teaching it fails to be properly responsive to these normative qualities of what's being taught.
Update: Could we be more explicit here, and say that legitimate instruction is precisely that form of teaching which is apt to produce knowledge? We can then pass the buck on the tough epistemological questions, whilst plainly distinguishing the history teacher from the sunday school instructor. However the details may go, those suspicious of religious brainwashing presumably hold that the religious instructor is not apt to produce knowledge in her students (even if her teachings happen to be true).