"All things are lawful for me," but not all things are helpful. "All things are lawful for me," but I will not be enslaved by anything. 1 Corinthians 6:12 (RSV)
The Law that St. Paul touches here are the Commandments of God revealed to the Children of Israel. The pious Jew intends to keep God's Law as a devotion to God's faithfulness and obedience for daily life.
As St. Paul wrote in his letter to the Romans, the Law is good. God's Law is a gift from the Father and should be seen as such. St. Paul is also clear that the Law cannot make us righteous before God. We are justified by grace through faith, apart from works of the Law. However, the good Law of God still stands.
In Christ, we are free from all the Law demands. As St. Paul notes, all things now are lawful. If we were able to live without sin, our freedom in the law would be absolute. However, sin still is at work in us, and thus, though all things might be lawful, our sin would cause a good thing to be a danger for us.
We must take care so our freedom in Christ does not become new slavery. As long as we live in this flesh, we are prone to temptation and sin. We could surrender Christ for the bondage of sinful things.
Luther reminds us that our Christian freedom is never merely for ourselves. We are set free to love and serve God and our neighbor. Loving the Lord and our neighbor is the fulfillment of the Law.