Originally from here.
The word antinomianism comes from two Greek words, anti, meaning “against”; and nomos, meaning “law.” Antinomianism means “against the law.” Theologically, antinomianism is the belief that there are no moral laws God expects Christians to obey. Antinomianism takes a biblical teaching to an unbiblical conclusion. The biblical teaching is that Christians are not required to observe the Old Testament Law as a means of salvation. When Jesus Christ died on the cross, He fulfilled the Old Testament Law (Romans 10:4; Galatians 3:23-25; Ephesians 2:15). The unbiblical conclusion is that there is no moral law God expects Christians to obey.
The apostle Paul dealt with the issue of antinomianism in Romans 6:1-2, “What shall we say, then? Shall we go on sinning so that grace may increase? By no means! We died to sin; how can we live in it any longer?” The most frequent attack on the doctrine of salvation by grace alone is that it encourages sin. People may wonder, “If I am saved by grace and all my sins are forgiven, why not sin all I want?” That thinking is not the result of true conversion because true conversion yields a greater desire to obey, not a lesser one. God’s desire—and our desire when we are regenerated by His Spirit—is that we strive not to sin. Out of gratitude for His grace and forgiveness, we want to please Him. God has given us His infinitely gracious gift in salvation through Jesus (John 3:16; Romans 5:8). Our response is to consecrate our lives to Him out of love, worship, and gratitude for what He has done for us (Romans 12:1-2). Antinomianism is unbiblical in that it misapplies the meaning of God’s gracious favor.
A second reason that antinomianism is unbiblical is that there is a moral law God expects us to obey. First John 5:3 tells us, “This is love for God: to obey His commands. And His commands are not burdensome.” What is this law God expects us to obey? It is the law of Christ – “Love the Lord your God with all your heart and with all your soul and with all your mind. This is the first and greatest commandment. And the second is like it: Love your neighbor as yourself. All the Law and the Prophets hang on these two commandments” (Matthew 22:37-40). No, we are not under the Old Testament Law. Yes, we are under the law of Christ…
Antinomianism is contrary to everything the Bible teaches. God expects us to live a life of morality, integrity, and love. Jesus Christ freed us from the burdensome commands of the Old Testament Law, but that is not a license to sin, but rather a covenant of grace. We are to strive to overcome sin and cultivate righteousness, depending on the Holy Spirit to help us. The fact that we are graciously freed from the demands of the Old Testament Law should result in our living our lives in obedience to the law of Christ. First John 2:3-6 declares, “We know that we have come to know Him if we obey His commands. The man who says, ‘I know Him,’ but does not do what He commands is a liar, and the truth is not in him. But if anyone obeys His word, God’s love is truly made complete in him. This is how we know we are in Him: Whoever claims to live in Him must walk as Jesus did.”