How do you differentiate between persuasion and manipulation?
The difference lies in the means we use to persuade. The Word of God is the only legitimate means of persuasion. Legitimate persuasion is cognitive—stirring the mind with reasonable truth. Convincing with tear-jerking stories, histrionics, and emotional outbursts takes an unfair advantage of people and wrongly muddles their thinking. That does not mean we cannot use all the communication skills available to us, but we should avoid playing on people's emotions, even by repeated singing or playing of hymns. These are artificial and should be avoided because they bypass the reason.
Our goal in preaching is to constrain people to choose change because it is reasonable and right before God, not because they have been manipulated into some momentary feeling or action. We persuade them from the Scriptures to choose the right course of action. We do not pile on emotional pressure until they break. We want them to know clearly what the alternatives are and that they must choose.
If after hearing our sermon someone does not know what he is supposed to do about it, we did not reach that person. I believe the legitimate point of persuasion ends with the clear presentation of the truth and must not move beyond that to artificial emotional stimuli for eliciting a response. This latter kind of appeal has produced false Christians and weak believers bouncing from one emotional high to another without a theology to live by.
In 1 Tim. 4:13, Paul writes to Timothy, "Until I come, give attention to the public reading of Scripture, to exhortation and teaching." What he tells Timothy is to read the text, explain the text, and apply the text. That verse is a call to persuasive, expository preaching.
Paul himself was a very persuasive preacher, but he never tried to manipulate emotions to move people artificially. At the end of one of his messages, King Agrippa exclaimed, "In a short time you will persuade me to become a Christian" (Acts 26:28). Agrippa clearly understood the message. Sadly, he made a wrong decision in spite of his understanding.
Ultimately, however, our sermons will only be as persuasive as our lives. A traveling speaker who does not remain in one place long enough for people to get to know him may be able to "fake" it without a consistent life to back up his message, (though this is regrettable). Those of us who preach to the same people week after week, however, cannot do that. Our people know us, and our persuasiveness depends on the quality of our lives.
Paul's preaching was persuasive; but it was his life that won the hearts of people. The Ephesian elders cried when Paul left them, but not because they would not hear him preach anymore. They were "grieving especially over the word which he had spoken, that they should see his face no more" (Acts 20:38). The integrity of the preacher's life is a key element in persuasiveness.
For a insider's look into John's pastoral heart, consider the audio series Insight into a Pastor's Heart, a rare opportunity to hear what makes him tick.