Over the years, I've had the opportunity to attend several church planting conferences.  Often, a session discusses whether preacher should preach topically, by text or if it should be expository.  What form of preaching is best, let's look at the options:

Topical Sermons: A topical message is a sermon where a preacher uses passages to support a thesis about a particular topic.  (eg. How to be a Godly Husband)  At a Purpose Driven Church Planting Conference,  one of the presenters said the very act of preaching expository sermons is a contradiction to the Bible.  To preach expository sermons was 'unBiblical' in the sense that the books were written to deal with specific issues & topics.  This is the model that a preacher should preach today.  Paul & Peter wrote topicaly and used 'texts' to support their points.

Text Sermons: A text message is one where a singular passage of the Bible is used as a jumping point to discuss a particular thesis. (eg. 1 Corinthians 13 to discuss Love is an Action)

Expository Sermons: An expository sermon is following a book of the Bible, passage-by-passage to allow the text to determine the point.  (eg. Galatians 1:1-4)  Most conservative churches would argue that expository preaching is the only way to preach.  Reformed churches stress lectio continua (preaching through whole books of the Bible in course).   Timothy Keller summarizes the sentiment as he writes why conservative churches feel non-expository preaching is theologically inferior; "1) First, other forms of preaching are considered 'man-pleasing' because we are choosing texts we prefer rather than preaching through the 'whole counsel of God' as God provides it in the Bible.  2) Second, other forms of preaching are more open to abuse since your thesis is not being controlled directly by the text.  3) Thirdly, other forms of preaching do not show as much honor to the text of Scripture.  The expositor focuses on the Biblical passage itself in a way that the others do not."

Haddon Robinson argues that expository preaching is essential in a postmodern context:

Twenty years ago it would have been almost impossible to bring a case to court against a minister. Today a lawyer that's defending a minister will do every thing that he can to keep the people in the jury from thinking of him as a minister. So we have lost a lot of the base, for a lot of different reasons. What we are really trying to say is, "O.K. if I can get people to study the Bible and to see the text, I believe that the Bible is self-authenticating." If I can get you to really read it, to look at it, to hear it, to understand it, it has its own power to convince and to convict and to change people.

Therefore in a postmodern age one reason that we work with the biblical text is to have the authority of the text — and behind that the authority of God — behind what we say. I've always believed that, but it has become clearer to me now than it has been in the past. That is not to say that the person in the pew has to accept my view of inspiration. It is simply to say that if the Bible is what I believe it to be — the word of God — and that the Spirit of God answers to the Word, then if I can lay that out before them in a relevant fashion it has the power to do what my authority today can't do. (source: Expository Preaching in a Narrative World: An Interview with Haddon Robinson)

Is there only one way to preach?  Tim Keller states that ultimately the method you chose is going to be selected for practical purposes.  (Often personal reasons & convictions that drive this.)  At the end of the day, preachers are going to tend to gravitate to what they are most comfortable with.

[Triperspectival Aside:  I would argue that Kings tend toward topical (and Purpose Driven Church plants), Priests toward text and Prophets toward Expository as part of their natural personality/gift structure.]

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