“Law is an ordinance of reason for the common good, promulgated by the one who is in charge of the community” (St. Thomas Aquinas, STh I-II, 90, 4; CCC 1976). Law is primarily a reasonable plan of action, “a certain rule and measure of acts whereby man is induced to act or is restrained from acting” (S.t., 1–2, q. 90, aa. 1, 3; S.c.g., 3, 114).
Eternal Law is the Divine Wisdom of God which oversees the common good and governs everything. Eternal law is God’s plan to lead all creation towards God’s eternal salvific plan to be holy and blameless before Him through Jesus Christ (Ephesians 1:4–5). God, as “Being-itself”, is able to promulgate such a law as God the Creator’s reason is also perfect wisdom. Everything in nature reflects the Eternal Law in their own natures (S.T. I-IIae, 91, 2). Things act according to their nature, so they derive their proper ends (final cause) according to the law written into their nature.
Divine Law is the historical laws of Scripture given to us through God’s self-revelation. Divine law is divided into the Old Law and the New Law, which correspond to the Old and New Testaments of the Bible (q91, a5). The Old Law, revealed by God to Moses, “is the first stage of revealed Law. Its moral prescriptions are summed up in the Ten Commandments” (CCC 1962). It has an extrinsic focus — motivated by fear — and promises earthly rewards (such as social peace). It expresses immediate conclusions of the natural moral law.
The New Law perfects the Old Law. The New Law, through the teachings of Jesus — commands internal conduct — and reaches us by divine love — promising love and heavenly reward. The New Law “is the Holy Spirit given through faith in Christ, which heals and is expressed through love.” It gives interior strength to achieve what it teaches. It is also a written law found in Christ’s teachings (in the Sermon on the Mount, the Beatitudes, etc.) and in the moral catechesis of the apostles, summed up in the commandment of love.
Natural Law is “the rational creature’s participation in the eternal law” (ST I-II, Q. 91, A. 2.). “The highest norm of human life is the divine law — eternal, objective, and universal — whereby God governs us according to His wisdom and love. God makes Man a sharer in His law so Man can recognize the unchanging truth” (DH 3). The natural law “hinges upon the desire for God and submission to Him, as well as upon the sense that the other is one’s equal” (CCC 1955).
It is “natural” as it consists of Reason given to us by the “higher reason” of the divine Lawgiver. They are natural as they are objective principles which originate in human nature (GS 16; DH 14). The natural law is universal because it encompasses every person, of every epoch (cf. CCC 1956): “it is immutable and permanent throughout history; the rules that express it remain substantially valid” (CCC 1958).
Every man is bound to live by his rational nature, guided by reason. The natural law expresses the dignity of the person and determines the basis for his fundamental rights and duties (CCC 1956, 1978). The first principle of the natural law is “good is to be done and pursued, and evil avoided” (q94, a2, p. 47; CCC 1954). All other precepts of natural law rest upon this. The Church, through its Magisterium, is the authentic interpreter of the natural law (cf. CCC 2036). Since mankind is subject to sin, grace and Revelation are necessary for moral truths to be known “by everyone with facility, with certainty and no error.”
Human Law is the interpretation of natural law in different contexts (ST II.I.95–97). Natural law is a foundation for moral and civil law. Government laws are dictates of practical reason from the precepts of Natural Law.
Law is not about individual morality. Individual vices should be legislated against when they threaten harm to others. Rulers of the State should take the general moral precepts of nature and specify them into State laws, e.g., the repugnance of murder is legislated into punishments.
Hierarchy of Law
For Aquinas, human laws are derived from natural law which is a participation in the eternal law. Therefore, eternal law is at the top, followed by natural law, and then human law. Divine law is the revealed law of God to man, while natural law is the imprint of eternal law on the hearts of men.
 Cf. St. Thomas Aquinas, Summa Theologiae , I-II, q. 108, a.1.
 John Paul II, Enc Veritatis splendor , 44.
 Pius XII, Enc. Humani generis : DZ 3876. Cf. Catechism , 1960.