1 Then the LORD spoke to Moses, saying, 2 “Speak to the children of Israel, and say to them: ‘I am the LORD your God. 3 According to the doings of the land of Egypt, where you dwelt, you shall not do; and according to the doings of the land of Canaan, where I am bringing you, you shall not do; nor shall you walk in their ordinances. 4 You shall observe My judgments and keep My ordinances, to walk in them: I am the LORD your God. 5 You shall therefore keep My statutes and My judgments, which if a man does, he shall live by them: I am the LORD. (Leviticus 18:1-5)
Clearly, the Bible teaches that we are not justified by the works of the Law: "Therefore by the deeds of the law no flesh will be justified in His sight," (Romans 3:20); "being justified freely by His grace through the redemption that is in Christ Jesus," (Romans 3:24); "Therefore we conclude that a man is justified by faith apart from the deeds of the law." (Romans 3:28); "Much more then, having now been justified by His blood, we shall be saved from wrath through Him." (Romans 5:9); "knowing that a man is not justified by the works of the law but by faith in Jesus Christ, even we have believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law; for by the works of the law no flesh shall be justified." (Galatians 2:16); "But that no one is justified by the law in the sight of God is evident, for 'the just shall live by faith.'” (Galatians 3:11); "You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace." (Galatians 5:4)
Clearly, justification is a gift from God. There is nothing, by way of works of the Law, that we can add to Christ's atonement, and the efficacy of His blood which was shed to satisfy the just wrath of God that is rightfully directed towards sinners because of their sin. You cannot be a Christian if you do not believe this fundamental and foundational truth. But this does not make the Law of God redundant, nor does it make justification by faith New Testament only. Believers have always been justified by faith: Noah was justified when He believed God about the impending flood; Abraham believed God about the need to have a son of faith sacrificed; Moses believed God when He spoke from a burning bush that was not consumed; David believed God in the face of enemies; the prophets believed God in the face of persecution; and so on, all the way down through the ages, from the Creation right up until today.
On the other hand, the evidence that these people believed God was that upon the foundation of their free and unmerited justification without the works of the Law, they confirmed their salvation by living out the good works which God prepared beforehand that they should walk in them, and it is the Law of God which defines what is a good work, and what is not. The Law of God is given by God as a grace gift. If, after being justified by grace, through faith, we then embrace the Law of God as God's measure of sanctification -- a measure of our journey along the path of becoming progressively more and more like Christ -- then our obedience to the commandments of God will bring us life. If you steal you will die, but if you work hard, give generously, and enjoy the prosperity that God provides as a reward for labour, then you will live. If you murder, you must die, but if you work to see others healed, speaking kind words where ever there is an opportunity, you will live. If you commit acts of homosexuality, fornication, adultery, bestiality, and so on, you will die, but if you enjoy life with the wife of your youth, you will live.
15 Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. 16 For all that is in the world—the lust of the flesh, the lust of the eyes, and the pride of life—is not of the Father but is of the world. (I John 2:15-16) This passage from I John echoes the passage recorded above from Leviticus. Those who are justified by grace through faith are to live a very different lifestyle to those who are of the world. If we claim to be justified, but live an unsanctified life, then we are liars and the love of the Father is not in us. If the love of the Father is not in us, then neither is the Spirit of His Son. If we have not the Spirit of Jesus, then we are none of Him - we are not justified, we are not saved, we are self-deceived.
Let the cry of justified King David rise in me, "Oh how I love Thy Law! It is my meditation all the day." (Psalm 119:97) Let me live by the exhortation of Jesus, "If you love Me, keep my commandments" (John 14:15) "If you keep My commandments, you will abide in My love, just as I have kept My Father’s commandments and abide in His love." (John 15:10) Amen!