Galatians 5:1, For freedom Christ has set us free; stand firm, therefore, and do not submit again to a yoke of slavery.
Freedom from what? Freedom from the Law. This is why Luther says the Law no long applies to the Christian, because we are no longer under Law, but grace (Romans 6:14). That is, the Law is no longer necessary for us to keep for salvation—for righteousness and holiness. This is because Christ has kept the Law for us, through whom God the Father declares us righteous, and who gives us His Holy Spirit to make us holy. Yet we do the Law because it pleases us to do the Law for the sake of our neighbour, for Christ has given us the freedom to love God and the freedom to do the Law without coercion.
Therefore, to require the Law as a salvific necessity for the Christian is to put oneself under the yoke of slavery once again; for before Christ, we were slaves to the Law. Through Christ, we are no longer slaves to the Law, but slaves to righteousness (Romans 6:15-23). Again, a slave to righteousness means we are free to love God and free to do the Law out of pleasure rather than fear, for it is Christ’s commodious yoke of grace we now carry.