When Paul says "we are not under law but grace", he is not saying the moral law has no relevance to the life of a believer. Rather he is stressing the fact we are not under the law AS A COVENANT, but rather we are under the COVENANT OF GRACE.

To the gentiles that was the Covenant of Works to the Jew that was the Mosaic Law in addition. God relates with humanity through out redemptive history via covenants. The moral law which has been God's standard for human living since the garden is the very standard ...

Jesus never fell short of but lived out perfectly. The standard and covenant which Adam broke, the better Adam (Jesus Christ) kept perfectly on behalf of his people. The righteousness which is imputed to us through faith is that of Christ's, obtained by his perfect obedience.

The Law in itself didn't have the power to bring about obedience but the Spirit received by virtue of the CoG does. If we are to be imitators of Christ we must desire to live like him, which is according to God's moral law.

Being empowered by the Spirit we can obey the moral law, not in order to be justified but having already been justified. It is confusing to hear those who profess Christ believe the 10 commandments has no relevance/usefulness to the life of a believer.

This isn't legalism, this is just historic biblical Christianity.