In a blog post, Justin Buzzard wrote:

While I think it is important to be known more for what you are for than what you are against, just a cursory reading of the Bible shows that it also calls us to deal with false teaching. Why? Because false teaching is dangerous and destructive; it hurts people.

About ten years ago I heard Ben Patterson say something that I will never forget. Ben told the story of a retired pastor who began noticing that his former congregation was sliding away from orthodoxy. The pastor saw this as his fault, noting the one thing he thought he did most poorly as a pastor. The pastor stated, in two sentences, his great failure as a pastor: "I always told people what to believe. My great mistake is that I never clearly taught my people what NOT to believe."