In the case of the Galatian churches, it was the idea that Gentile believers now had to be circumcized under the Law of Moses in order to be complete Christians. If you let yourselves be circumcized in obedience to the law of Moses, then Christ will be of no value to you. He will have died for nothing. You will have fallen from grace. You will again be in slavery under the Law, and the Law will kill you.
Every road has two ditches, and the road of freedom to salvation is no exception.
The ditch on the right side of the road, which I just described, is the ditch of legalism - using the law to get on God's good side. That was the ditch that Luther and the Reformers struggled to steer clear of. The ditch on the left side of the road, is called license . It's the opposite of legalism. Many a good Lutheran, trying to avoid the ditch of legalism has driven into this one, both in Luther's day and ours.