Two things should be stated first and foremost. First, it is impossible for a human to know the heart of another human. That is God’s province alone. Therefore, while we can safely determine what would be considered as living under the law, we cannot determine whether another Christian is truly living under the law or is simply weaker in their faith and feels more comfortable resting in the boundaries of the law. If a person feels that they can earn their salvation or earn God’s love through following the law, then they are operating outside of God’s grace. If a person loves God’s law with all of their hearts, and simply chooses to follow the law as best they can, then they are entirely within the bounds of Christianity. It is not reverence for Mosaic Law that is wrong, but relying on the Mosaic Law to earn our salvation.
Consider the example of Messianic Congregations (I was a Messianic for a time). They observe the Mosaic Laws, but they do so ONLY out of respect for God. Otherwise, there is no clinging to the law for salvation. Christ was our final sacrifice for sin. So while it is outside of God’s will for us to rely on the Law to save us, it is entirely within the bounds of our salvation to love God and to love His Law. Psalm 119 is a beautiful picture of devotion to the Law that would help many Christians to gain a better understanding and love for the Old Covenant.