Expository means that preaching aims to exposit, or explain and apply, the meaning of the Bible. The reason for this is that the Bible is God’s word, inspired, infallible, profitable—all 66 books of it. The preacher’s job is to minimize his own opinions and deliver the truth of God. Every sermon should explain the Bible and then apply it to people's lives. The preacher should do that in a way that enables you to see that the points he is making actually come from the Bible. If you can’t see that they come from the Bible, your faith will end up resting on a man and not on God's word.
John Piper