There is a clear and present danger that the devil may take away from us the pure doctrine of faith and may substitute for it the doctrines of works and of human traditions. It is very necessary, therefore, that this doctrine of faith be continually read and heard in public. No matter how well known it may be or how carefully learned, the devil, our adversary, who prowls around and seeks to devour us (1 Peter 5:8), is not dead. Our flesh also goes on living. Besides, temptations of every sort attack and oppress us on every side. Therefore this doctrine can never be discussed and taught enough. If it is lost and perishes, the whole knowledge of truth, life, and salvation is lost and perishes at the same time. But if it flourishes, everything good flourishes—religion, true worship, the glory of God, and the right knowledge of all things and of all social conditions.
— Martin Luther, Luther’s Works, Vol. 26: Lectures on Galatians, 1535
(Note: There are very few commentaries that I would ever recommend a Christian take up and read cover to cover just for edification; however Luther's great Galatians commentary is one that every believer should purchase and read slowly, from beginning to end. It is a devotional work of such depth and richness that you will not be disappointed. I promise.)