From the TeachingtheWord Bible Knowledgebase

Some people and churches use a snippet of Romans 6:14 and 15 - "we are not under law but under grace" - to say that the Law of God as given in the Old Testament has no place in the New Testament church.¹ But when we truly understand what the Bible says about the nature and purposes of God's Law, we find that this is not true.

Three Kinds of Law

First, we need to understand that there are three kinds of law in the Old Testament. First, there is the moral law of God, embodied in the Ten Commandments of Exodus chapter 20. We see the moral law in God's command to our first parents, and the physical and spiritual death and the disorder in nature that resulted from their disobedience. The moral law of God is written on the heart of every human being because we are created in the image of God. As both the Westminster and London Baptist Confessions of Faith so well put it, Scripture tells us that God requires "personal, entire, exact, and perpetual obedience" to His moral law. (See, for example, Genesis 1:26-27, 2:17; Job 28:28; Romans 2:14-15, 5:12, 5:19; Galatians 3:10-12; James 2:10.)

Second, God gave the ceremonial law to the nation of Israel, which regulated the worship of God and established a sacrificial system whereby men could bring the blood of an innocent animal as an atonement for their violations of God's moral law. The ceremonial law prefigured the person and work of Christ, who would come to make full and final atonement for sin, and this law was completely fulfilled in Christ. (See, for example, 1 Corinthians 5:7; Galatians 4:1-3; Ephesians 2:15-16; Colossians 2:14-17; Hebrews 9:1-10:1)

Third, God gave Israel a civil or judicial law for the government of the people, dealing with matters of crime and punishment, regulation of land ownership, marriage and divorce, indentured servitude, and so on. This law applied only to the nation of Israel, although it embodies the principles of God's moral law in practical application. (See, for example, Exodus 21:1-22:29.)

The Use of the Law in the New Testament

God's law has two purposes under the New Covenant. First, God's moral law shows sinners their need of salvation. The law is God's perfect standard of obedience (Deuteronomy 5:32, James 2:10-12). Scripture demonstrates that every human being has broken God's law (e.g., Isaiah 53:6). The law condemns all men as sinners subject to the wrath of God, and without hope apart from Christ (Romans 1:18ff, 2:12-16, 7:1-12, 10:4). The law is our schoolmaster (tutor) to bring us to Christ, but not in order that we might obey the law to be saved, but in order "that we may be justified by faith" (Galatians 3:24).

The Old Testament ceremonial law is rightly included in Gospel preaching, not as a way of salvation, but to show the extent of man's sin problem, man's inability to save himself, and the richness of salvation in Christ.

The second purpose of the moral law of God is in the life of the believer in Christ. The moral law of God is the believer's rule of life, not as a means of earning merit with God, but out of the motivation of love for Christ as Savior (John 14:15, Romans 3:31, 13:8-10). Because we are freed from the condemnation of the law and have the indwelling of the Holy Spirit, the commandments of God are no longer burdensome for the believer (1 John 5:3).

This brings us back to that passage in Romans 6, which speaks of believers as being "not under law but under grace." What is the proper understanding of that phrase? As we've emphasized many times before, we always need to look at Scripture in context. And as we look at the phrase "not under law but under grace" in context, we see that God is saying that the fact that we have been freed from the condemnation of the moral law by Christ's substitutionary atonement, does not abrogate God's requirement that we obey His moral law as our rule of life in Christ:

"Therefore do not let sin reign in your mortal body, that you should obey it in its lusts. And do not present your members as instruments of unrighteousness to sin, but present yourselves to God as being alive from the dead, and your members as instruments of righteousness to God. For sin shall not have dominion over you, for you are not under law but under grace. What then? Shall we sin because we are not under law but under grace? Certainly not! Do you not know that to whom you present yourselves slaves to obey, you are that one's slaves whom you obey, whether of sin leading to death, or of obedience leading to righteousness? But God be thanked that though you were slaves of sin, yet you obeyed from the heart that form of doctrine to which you were delivered. And having been set free from sin, you became slaves of righteousness." (Romans 6:12-18)

References:

1. Some churches and seminaries even exclude or diminish the authority of the Old Testament in their doctrinal statements. For example, the Cincinnati Christian University Statement of Faith, point 7, says that "The basis of unity of all believers is a commitment to the authority of the New Testament" (emphasis added, https://www.ccuniversity.edu/about/statement-of-faith).

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