The preaching of the gospel is not brainwashing. It is building the case for Christ over a period of time. Paul did it to the Ephesian Jews (Acts 19). He reasoned with them until it was clear to him that they were not going to believe the gospel he was preaching. Paul gave them the freedom to accept or reject his message.
Brainwashing, on the other hand, attempts to weaken the critical facilities of the audience, wearing them down, physically and mentally, until they just don’t care to think critically any more. Religious cults do this.