I'm going to make this as clear and succinct as possible: the law is on earth and grace is in heaven.
You are not justified by the works of the law, but the law is holy, and the commandment holy, and just, and good.
As far as justification: grace and only grace. Come boldly before the throne of God. Grace is in heaven.
As far as knowing good and evil and living righteously, the law. Sin on earth, and suffer the consequences on earth. Obey on earth, and reap the rewards of obedience. The law is on earth.
In the O.T. there are civil, ceremonial and moral laws. As the Temple and priesthood were only temporary and have ended, we are no longer to observe the ceremonial law in outward fashion, but the spiritual significances thereof are eternal. Some civil law still applies, some doesn't. Depends on whether or not they have their roots in ceremony or morality. Still, all civil law has a spiritual significance to the church. The moral laws are eternal, universal and are to be observed and enforced world wide, Christian or not.