A teacher of scripture who wishes to teach and preach correctly needs to present all the different doctrines of the Bible correctly. If this isn’t a heavy expectation, one cannot call themselves a good and correct teacher unless they also understand how to balance and handle God’s two words in the Bible… His words of Law and His words of Promise.
In other words, it is good if we are able to distinguish the difference between Law and Gospel. We can confidently share with another that God’s demanding word of Law issues threats, reveals sin and the like. We can also boldly share that God’s promising word of Gospel grants assurance, absolves sin and the like. However, if we apply the Law and Gospel in a manner that ‘mingles’ them and confuses them, we essentially poison other people’s souls.
For example: God’s demanding words of Law are only to be proclaimed to people who are secure in their sin, people who feel as if they have it all together with their own spiritual accomplishments. The demanding words are to be applied so as to ‘rattle their cage’ and bring them to the end of themselves. Once they are rattled, God’s promise words are to be applied so as to grant them assurance in Christ. Just think of the damage that is done when this is mixed? Giving God’s demanding words when someone is already spiritually bankrupt? That is enough to drive the poor person to unnecessary despair and spiritual suicide! Giving God’s promising words to someone who is spiritually puffed up? That is like giving precious pearls to filthy swine!
To make things worse we can easily mingle God’s demanding words and God’s promise words. Ever hear a sermon full of good advice? Is this the Gospel or is this Law? It clearly is Law, simply disguising itself as Gospel. Ever hear a sermon stating the demands of the Gospel? What is going on hear? This again is Law disguising itself as Gospel.
Thus a correct teacher of scripture not only understands the doctrines of scripture and the definitions of Law and Gospel, but also applies and frames them in their proper context. This is an art that is learned in the school of experience.