What is the relationship of the Christian saint to the Law of Moses?...

He is dead to the Law of Moses. The Law of Moses, including all aspects of the first covenant, retains no authority over the Christian.

However this does not mean we are free to behave unrighteously. It does mean we have changed from one set of spiritual requirements to a different set.

Two facts have been established:

There never is one moment when we are not under one covenant or the other—we never are without responsibility in the sight of God.

The two covenants cannot be mixed. The first covenant consists of a body of obligations and restrictions that we are to do. The second covenant is Christ.