What is the purpose of the Law under Moses? According to the New Testament, it’s good but it can’t save you. It’s from God, and so it’s good, just as all of God’s gifts are good. And following the Law was the way God’s people could demonstrate that they were God’s people. But a person didn’t “get saved” by following the Law, even in the Old Testament.
A lot of Christians don’t understand this. We like easy dichotomies: in the Old Testament, people were saved by following the Law; in the New Testament, they are now saved by Jesus. But that’s a false dichotomy and bad theology.
The Law provided the sign and the boundaries of kingdom life. We don’t follow the Law to GET saved, we follow the Law because we ARE saved–just as under the covenant of Christ, we don’t do good works to get salvation; we do good works because we have salvation. Works, then and now, are the result of kingdom citizenship, not the keys into the kingdom.