I think this is the key. Paul's not just saying we shouldn't obey the laws of Moses for salvation, he's saying we shouldn't obey even as a matter of obedience, as it puts us under the whole of the law.
If we don't eat pig, or shellfish or something because it's healthier, or we don't serve them up because we don't want to offend our Jewish/Muslim guests, that's fine (right?). But as soon as we start doing such things to try to obey the law of Moses (not even for salvation), it puts us under the whole of the law.
Or have I misunderstood?
This will be my last post here on this topic, because it is irrelevant to the thread's topic. My understanding of his words is different. The unbelieving Jews tried to remain perfect and righteous by means of the Law. But if you transgress any part of the Law (which all have done) then you are a sinner. To go on trying to remain perfect and righteous is a useless act.
Paul was saying that if sinners continued to try to remain perfect and righteous by observing the Law then 1. not only was Christ to no effect because they had their faith in the Law rather than Christ but 2. the Law actually condemned their past sins and therefore condemned them rather than saved them. It's pointless to try to earn salvation and righteousness by obedience to the Law rather than faith in Christ. The Gospel is about faith in Christ. Paul was saying that these people had put their faith in the Law rather than putting their faith in Christ. Therefore they were unsaved. It has nothing to do with obedience for the sake of obeying the Law. It's about where one's faith is.
Those who put their faith in the Law to save them
are subject to obey the whole Law. Otherwise one slip and the Law automatically condemns them. Paul says nothing on obeying the Law as a matter of obedience. It's all about where one's
faith is. Let's go through it step by step.
1. X trusts that they can earn salvation by a perfect life and good deeds (this is called putting your faith in the Law).
2. Therefore X is not trusting in Christ's sacrifice for them (this is called being cut off from Christ).
3. X transgresses the Law once.
4. The Law condemns X.
5. Since X has no faith in Christ and the Law condemns X they are unsaved.
This has nothing to do with obedience. It's a matter of one's faith. Put your faith on Christ
our Rock and even though you fail to obey the whole Law you will not be condemned by it. However, you can still try to obey the whole Law as a matter of simple obedience and willingness to walk a godly path.
The only reason you would be subject to obey the whole Law is if you are not trusting in Christ but rather the Law for salvation.