"For a sermon to be genuinely biblical, the text [of the Bible] must set the agenda as the foundation of the message. Scripture should be central. Expository preaching [i.e. preaching based on a biblical text] does not make statements and then look to the Bible for support; it begins by examining the Scriptures." (Planting Missional Churches, p. 272)
I agree. A sermon is not biblical unless the Bible itself sets the sermon's agenda. But that is not all. After the biblical text is explained, it needs to be illustrated, helping those listening understand what the God-driven truth its revealing 'looks like' in their world. Finally, it needs to be applied in a compelling way which both unmasks our sin and drives us in hope to Jesus for forgiveness and holy power to more fully embrace His glorious purpose in making us.
When these three components converge: explanation of a biblical text, illustration of the text in contemporary terms and application of the text's gospel truth to our lives, then you have a truly biblical sermon.