Answer
The New Testament speaks very strongly about teaching false doctrine simply because God wants his people to understand the difference between truth and error. For instance, you have in 1 John chapter 4 John explaining the difference between the spirit of truth versus the spirit of error. And all throughout the New Testament practically every book of the New Testament has something to say about false teachers, false prophecies, false prophets, and whenever you find that, it typically is surrounding the person and work of Christ, who he is and what he did on the cross. So, if the New Testament gives that much evidence of speaking so strongly about false doctrines, it's apparently very important from God to his people that we understand the difference between truth and error.
Answer by Rev. Clete Hux
Rev. Clete Hux is Director of the Apologetics Resource Center (ARC) headquartered in Birmingham, Alabama. A Teaching Elder in the PCA, he has pastored churches in Alabama and South Carolina. He holds a M.Div. from Birmingham Theological Seminary (BTS) where he teaches Comparative and World Religions, Contemporary Cults and New Religious Movements, Apologetics and Personal Evangelism, Christianity and Competing Worldviews, and other related courses. He has also taught courses in Systematic Theology, Christian Ethics, Contemporary Theology, New Testament Theology, and Paganism in Contemporary Culture.