Every time we work through Galatians, I marvel over my students' response to Paul's idea that the law is not an ethical standard for his gentile converts. The harder Paul pushes for a law-free gospel, the more my students push back (well, they really don't push back too hard--honestly, they're in shock). It just doesn't seem right to their ears. I think their resistance is attributable to two sources: their religious upbringing and American culture (which of course are intrinsically linked).
They've been taught their whole lives that law is the benchmark for right behavior, whether at home or in church. Sin, therefore, is defined as breaking the law. Now, that makes sense for unbelievers; but for those of us who have been set free from the law of sin and death, it should be quite apparent what (or more accurately who) sets the standard for righteousness: Jesus Christ. To obey him is right, to disobey him is sin. It's as simple as that, which is why Paul believed the only way to live righteously is to "walk" in the power of Christ's Spirit, the Holy Spirit. What the law could not do--effect righteousness--God did through Christ. Our obedience has nothing to do with law (Paul insists our obedience is based on faith).
The other mental block to Paul's law-free gospel comes from growing up in America, where righteousness/justice is defined by law. Indeed, to the ears of American Christians, law-free sounds like an invitation to anarchy. Besides, to the American way of thinking, our justice is supposed to be a God-given gift (not only to us but to the whole world). Imagine how difficult Paul's new standard of righteousness sounded to his Jewish kinsmen, who rightfully believed their law came from God.
No wonder Paul got in trouble, then and now.