The moral law is the Justice inherent to the very Nature of God. And since God is unchanging and eternal, the moral law is unchanging and eternal.
Saint Thomas Aquinas: “Wherefore as the type of the Divine Wisdom, inasmuch as by It all things are created, has the character of art, exemplar or idea; so the type of Divine Wisdom, as moving all things to their due end, bears the character of law. Accordingly the eternal law is nothing else than the type of Divine Wisdom, as directing all actions and movements.”
Summa Theologica, I-II, Q. 93, A. 1.
Saint Thomas Aquinas: “So then no one can know the eternal law, as it is in itself, except the blessed who see God in His Essence. But every rational creature knows it in its reflection, greater or less. For every knowledge of truth is a kind of reflection and participation of the eternal law, which is the unchangeable truth, as Augustine says (De Vera Relig. xxxi).”
Summa Theologica, I-II, Q. 91, A. 2.