Thoughts from John Piper on relevance in preaching:
As a preacher, I think a lot about relevance. Why should anyone listen to what I have to say? Why should anybody care? Relevance is an ambiguous word. It might mean that a sermon is relevant if it feels to the listeners that it will make a significant difference in their lives. Or it might mean that a sermon is relevant if it will make a significant difference in their lives whether they feel it or not.
That second kind of relevance is what guides my sermons and my writing. In other words, I want to say things that are really significant for your life whether you know they are or not. My way of doing that is to stay as close as I can to what God says is important in his word, not what we think is important apart from God's word.
--Taken from Finally Alive by John Piper