First let me say that I agree that we are not under the law, as it relates to salvation. We are saved by grace alone and not by our works thus the law does not save us by following it. When we see the law spoken of as a school master (Gal 3:24) or in other such terms it is in relation to us before we become a child of God. It is the law that reveals our great need for the atonement provided by Christ on the cross. So, again as the law relates to us before we believe we are under it but that relationship does change once we believe.
After salvation God’s law is our guide to the life that most glorifies God and is empowered by His spirit. Paul in 2 Tim 3:16-17 speaks of the whole council of scripture being useful, not just some of it, and this would first relate to the Old Testament that Paul would have taught from. Of course it also applies to the New Testament but all too often the relevance of the Old Testament to the New Covenant believer is overlooked. This comes, I believe, from a wrong understanding of our relation to the law after salvation. God has given us His law word as a guide for the path we are to follow, a path that most glorifies Him. We follow the law not for ourselves, primarily, but to worship Him magnify His name in all the earth as we realize the sanctification God works in our lives.
Matthew 28:19-20 an oft quoted section of scripture says:
(19) Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: (20) Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
The call here is to teach the nations God’s commands that at the time of the writing of this passage would have been contained in the Old Testament and would also, necessarily, include what is in the New Testament as it came about. But, the call is not to simply teach about the commands but to teach to observe or follow them. The great glory in this is that as believers we are empowered by God to do just that. We may fail and struggle but by studying His word and by the power of His Spirit we are enabled to follow all God calls us to do in all His word, again, for His glory and His purposes.
It would be rightly called legalism to try and follow God’s law by our power. But to do so by His power is not legalism but is obedience to our Lord and Savior. We need to recognize this distinction with regards to position with the law if we are to correctly understand how we are to relate to it as believers today. The struggle should not be about what laws we are to follow but how can we follow all of God’s law, of course not the ceremonial law that pointed to Christ, for His glory
Charles Spurgeon in his sermon on The Allegories of Sarah and Hagar (Sermon 69) speaks on this distinction in how the law relates to us both before salvation and after and I think his words are helpful.
On the Law prior to Salvation
“The law is, so to speak, Jesus Christ's dog, to go after his sheep, and bring them to the shepherd; the law is the thunderbolt which affrighteth ungodly men, and maketh them turn from the error of their ways, and seek after God. “
On the Law after Salvation:
“What is God's law now? It is not above a Christian—it is under a Christian. Some men hold God's law like a rod, in terrorem, over Christians, and say, "If you sin you will be punished with it." It is not so. The law is under a Christian; it is for him to walk on, to be his guide, his rule, his pattern. "We are not under the law, but under grace." Law is the road which guides us, not the rod which drives us, nor the spirit which actuates us. The law is good and excellent, if it keeps its place.”
The law does not disappear once one is a believer but is simply in a different relationship to us. It is for the believer a path unto our feet and a path that we need to seek to follow diligently since it is a path set out by God first for His glory and secondly for our good.