The Apostle Paul writes in the Epistle to the Colossians:
Therefore, if you died with Christ from the basic principles of the world, why, as though living in the world, do you subject yourselves to regulations— “Do not touch, do not taste, do not handle,” which all concern things which perish with the using—according to the commandments and doctrines of men? These things indeed have an appearance of wisdom in self-imposed religion, false humility, and neglect of the body,but are of no value against the indulgence of the flesh.
The way in which Christians are “not under the law” is one of those famous disputes in New Testament studies, but this passage seems to make at least one thing clear, you do not gain mastery over the flesh by adherence to regulations and restrictions concerning temporal things. Neither eating, nor not eating in itself has any bearing on your spirit.
Now of course, this doesn’t mean that you can flaunt your decisions and freedom, not giving one fig what others think or how they will respond. The Apostle expects his people to abstain from things on a regular basis. But the point of sin or righteousness is not in the thing, the abstention, nor the law. It is in the spirit, motivation, and self-mastery of the Christian. Are they acting out of love, rightly esteeming the other higher than themselves, and pointing towards a net goal of godliness? Or are they just trying to prove a point, look nice and moral, or please other people’s expectations?
That is the conflict between the flesh and the spirit.
Your “flesh” is, at the end of the day, just as immaterial as your spirit. It’s simply the selfish appetite rather than the selfless one. Rules are for kids, and they will need lots of them. But that’s precisely because they are kids, because they are in training. The mature Christian will know that he has to navigate between tricky waters all the time and that a mere “Don’t do that!” is never really the answer.
Paul’s discussion does have to do with the particulars of the Jewish holiness code. That is true. But it isn’t as if he doesn’t push beyond those to principles. He does. In fact, the principles are the point. Get to the heart of the matter. In this way Luther is as correct today as he ever was.