Thomas Aquinas wrote about the Gospel as the New Law in his Summa Theologica (I-II, Q.108, 1):
“There are works which are not necessarily opposed to, or in keeping with faith that works through love. Such works are not prescribed or forbidden in the New Law, by virtue of its primitive institution; but have been left by the Lawgiver, i.e., Christ, to the discretion of each individual. And so to each one it is free to decide what he should do or avoid; and to each superior, to direct his subjects in such matters as regards what they must do or avoid. Wherefore also in this respect the Gospel is called the law of liberty: since the Old Law decided many points and left few to man to decide as he chose.”
This means that what is not specifically commanded or forbidden by the teachings of Jesus are left up to the individual or the individual community to decide. They are not referred back to the Old Law, because the New does not fill in the gaps left by the Old or appeal to the Old where it remains silent. Silence in the New Law means freedom for those in Christ.
Aquinas goes on:
“He acts freely who acts of his own accord. Now man does of his own accord that which he does from a habit that is suitable to his nature: since a habit inclines one as a second nature. If, however, a habit be in opposition to nature, man would not act according to his nature, but according to some corruption affecting that nature. Since then the grace of the Holy Spirit is like an interior habit bestowed on us and inclining us to act aright, it makes us do freely those things that are becoming to grace, and shun what is opposed to it.”
Our own nature is corrupted (by the Fall) and the habits that we live by incline us accordingly. The Holy Spirit dwells in us though, directing our habits, not toward our nature, but toward grace. The Holy Spirit does not repair the old human by the old law. The Holy Spirit creates a New Human by the New Law. The New Person lives by “the perfect law that gives freedom” (James 1.25).
It is for freedom that Christ has set us free. Let us live only by his law of liberty.