In this sermon, the speaker discusses the character and purpose of a Christian in the world. He emphasizes the importance of living as a servant of God and interacting with a world that does not follow His rule. The speaker uses the analogy of salt and light to describe the role of a Christian. Just as salt adds flavor and preserves, Christians are called to bring a positive influence and preserve righteousness in the world. Similarly, just as light illuminates and exposes, Christians are meant to bring knowledge and reveal the truth of God to those in darkness.