Who is involved in making and fulfilling God's covenant, and what part does man play?

God alone is involved in making and fulfilling His covenant: He gives it, carries it out, and completes it. Man has nothing to do with it, and human beings cannot touch or alter those covenants. The covenant rests entirely on God's own action and faithfulness, not on human effort or participation.

Les teaches that God is one and is the one and only one involved in a covenant. God gives it, He carries it out, and He will complete it, while man has nothing to do with it. Even man cannot touch those covenants. In the surrounding context, the promises to Abraham were given without the Law, almost 500 years before the Law, and the Law does not cancel them. If the Law could give life, righteousness would be by the Law, but no person can keep all Ten Commandments other than Christ Himself.

Galatians 3:20

Now a mediator is not a mediator of one, but God is one.

Did God use a mediator between two parties in making His covenant?

No, God did not use a mediator between two parties in making His covenant. The covenant promise to Abraham was spiritual, heavenly in origin, and God alone is mediator, instigator, and fulfiller; He alone is involved. The text says a go-between is not a mediator of one, but God is one.