OK I should probably expound on this...
Law without Gospel = Law.
Gospel without Law = Law.
This is my shorthand way of expressing one of the dangers of an improper distinction of Law and Gospel.
C.F.W. Walther, the founder of the Lutheran Church-Missouri Synod, and an expert in Luther's teachings, said, "Paging through Scripture before we know the difference between Law and Gospel makes us think it contains nothing but contradictions, even more than the Koran" and, "properly to distinguish Law and Gospel is so difficult and such a high art for the preacher as a Christian. In fact, this is the greatest skill a human being can acquire."
So first, maybe some working definitions:
"Law" is that category of Scriptural teaching which shows us the rules. It tells us what to do and not to do. It shows us, also, that we fail - constantly - to measure up to God's perfect standard. It has to do with punishment. The Law is the "bad news" of Scripture. The Law shows us our sin, always.
"Gospel" is the category of Scriptural teaching which stands in opposition to the Law. The Gospel, strictly speaking, is the good news of Jesus Christ and all he has does for us. But chiefly it's about his death and resurrection to pay for our sins. This strict Gospel plays out into and is connected with many of the other "good news" passages and promises. But all those are, ultimately, rooted in Jesus and his saving work for us.
We Lutherans understand the extreme importance of maintaining both Law and Gospel in all our preaching and teaching. We take great pains to distinguish and divide them, exploring the relationship between these two great teachings of Scripture.
But some take no such care. Some even teach, falsely, that we should not carefully balance and maintain the tension between Law and Gospel.
Some tilt to the Law. We might call this "works righteousness" or "legalism". The good news of what Jesus has done for you is minimized, or not even mentioned. Jesus becomes an afterthought to the real business of what YOU need to DO. Taking Jesus out of our preaching and teaching makes Christianity into nothing more than a religion of the Law... just like every other false religion of man. For with the Law alone we cannot be saved.
But some tilt to the Gospel. And while Walther rightly states that the "Gospel should predominate", that doesn't mean we shouldn't preach the Law, and forcefully so. A particular abuse here is anti-nomianism, or a teaching that the Law doesn't matter, or that it shouldn't be preached. That Christians don't need to hear the bad news, the condemnation, the stinging rebuke of God's Law. It's too much of a "downer". Some seek to preach and teach "only the good news", and divorce Law from Gospel.
So now to my point: When you remove the Law, you are not simply left with Gospel. For the Gospel apart from the Law is meaningless. "Jesus has forgiven your sins" means nothing without you knowing that you are a sinner, or what sin is. It's a solution to a problem we wouldn't understand. We need both the diagnosis AND the cure. We need to be shown our sin, to know our sin, to know our forgiveness.
But it's worse than that. The Gospel without Law is not just meaningless; it changes. No one would preach a bunch of nonsense about forgiveness while the question begs, "forgiven for what?" No one would talk about Christ crucified for sinners when there's no need for such a thing. So in churches where the true preaching of the Law is absent - a preaching of the Law with an eye toward the Gospel - in those churches, the Gospel becomes something else.
If Jesus didn't come to forgive sins, because there's no law, and no sin.... then why did he come? As an example for me to follow? That's just more Law. That's telling me what to do. Or some make him into a new law-giver. "He shows us the law of love". He said, "Love your neighbor", "turn the other cheek". He told us to care for the poor and downtrodden. Jesus is about social justice! And so the lack of Law leaves us not with the Gospel - but a meaningless gospel. And guess what fills the vacuum? More Law!
Our sinful nature loves the law - in a way. And so anytime we can migrate to it, dwell upon it, our Old Adam is happy. But it's usually a warped law. A law that leaves us with a false sense of security. A law created in our own fashion just so that we can fulfill its demands. We convince ourselves, like the rich young man, "all these I have kept from my youth".
But a true view of the law leaves us in despair. It pulls our spiritual pants down and embarrasses us with our total helplessness. We can't save ourselves. We can't do anything right. We sin ALL THE TIME. Poor, miserable sinners.
And then, and only then, are we ready for the true Gospel. What unspeakable joy to go from the depths of the Law's despair to the heights of the Gospel's grand promises. We've all heard about people who have a close shave with death, or a terrible disease from which they recover. They have a "new lease on life". That's what happens, in spiritual terms, with Law and Gospel. Only when brought low by the Law can we be raised by the Gospel.
While Jesus certainly did speak the Law, and raised the bar in many ways, his main message was one of forgiveness for our sins. Jesus preached and taught BOTH Law and Gospel, too! So should his faithful pastors and teachers. So should his faithful church. So should his faithful people believe his words of both Law and Gospel, never compromising or diminishing either.