“Throughout the history of the church the greatest preachers have been those who have recognized that they have no authority in themselves and have seen their task as being to explain the words of Scripture and apply them clearly to the lives of their hearers. […]
Essentially they stood in the pulpit, pointed to the biblical text, and said in effect to the congregation, “This is what this verse means. Do you see that meaning as well? Then you must believe it and obey it with all your heart, for God Himself, your Creator and your Lord, is saying this to you today!”
Wayne Grudem – Systematic Theology
i agree. i do think it is possible to combine expository and topical, but i think the central part of the sermon should be the text.