Romans 6:14 For sin shall no longer be your master, because you are not under the law, but under grace.
We are not longer under the law, but which law is Paul referring to? He mentions a number of different laws:
1.) The Law of God (Romans 3:31, 7:22-25, 8:7)
2.) The Law of Sin (Romans 7:23-25)
3.) The Law of Sin and Death (Romans 8:2)
4.) The Law of the Spirit of Life (Romans 8:2)
5.) The Law of Faith (Romans 3:27)
6.) The Law of Righteousness (Romans 9:31)
7.) The Law of Christ (1 Corinthians 9:21)
In order for sin to no longer be our master, that would mean that at one time we were under sin. The error comes in not understanding that the first half of Romans 6:14 is talking about the same thing as the second half. This law which we are no longer under has something to do with no longer being under sin. If Paul was referring to the Law of God in verse 14, then that would mean that Paul was equating the Law of God with being sin, but in Romans 7:7 he clearly stated that the Law of God is not sin. Rather, it is through the Law of God that we define sin (1 John 3:4), so the Law of Sin would be instructions against following the Law of God. It is sin that leads to death (Genesis 2:16, Romans 5:12), so following the Law of Sin leads to death, or the Law of Sin and Death. According to Romans 6:8-9 death no longer has dominion over us, so in the context of not being under the law, sin and death no longer have dominion over us. So in Romans 6:14, Paul was introducing the concept of the Law of Sin and Death and was not speaking about the Law of God. When we come into faith, our Messiah's death allows us to be free from the bondage of sin and to overcome death, thus we are no longer under the Law of Sin and Death, but under grace. The Law of Sin is the exact opposite of the Law of God because sin is breaking the law:
Romans 7:21-23 So I find this law at work: Although I want to do good, evil is right there with me. 22 For in my inner being I delight in God’s law; 23 but I see another law at work in me, waging war against the law of my mind and making me a prisoner of the law of sin at work within me.
Romans 8:1 Therefore, there is now no condemnation for those who are in Christ Jesus, 2 because through Christ Jesus the law of the Spirit who gives life has set you[a] free from the law of sin and death.
Grace is receiving forgiveness for sin, which is breaking the Law of God, and sin leads to death, so being under grace means that we are free from the bondage of sin and death that result from it. If there is no Law of God, then there is no sin, and if there is no sin, then there is no need for grace. To not be under the Law of God is to be under the Law of Sin.