Moral law is a divine work of Wisdom that prescribes the ways to beatitude and proscribes evil, rooted in a rational order established by God for the common good (CCC 1950-1951). It finds its ultimate truth in the eternal law, which is declared and established by reason as a participation in God’s providence (CCC 1951). This includes the natural law, which is universal, immutable, and eternal, written on every human heart by God to enable the discernment of good and evil through reason (CCC 1954, 1956). As a fatherly instruction from God, the moral law reflects His higher reason and serves as the foundation for building human community and civil law (CCC 1954, 1975).

In the Old Testament, the moral law was revealed as a gift bestowed from within the covenant between God and His people (CCC 2076). The Decalogue, as stated in Exodus 20:2-17, provides fundamental and immutable duties toward God and neighbor that bind all people. While the Old Law served as a preparation for the Gospel, it was revealed because humanity often failed to read these truths in their own hearts (CCC 1980-1981).

Jesus Christ brings the moral law to its fullness and unity, acting as both the end of the law and the way of perfection (Romans 10:4, CCC 1953). He did not come to abolish the law but to fulfill it in its entirety (Matthew 5:17-19). Jesus refined the understanding of the law by shifting the focus from mere external actions to internal dispositions and intentions (Matthew 5:21-22), teaching that true righteousness surpasses that of the scribes and Pharisees through interior conversion and charity (Matthew 5:20). This New Law reforms the heart through faith, hope, and charity, summing up the moral life in the Golden Rule and the new commandment to love others as Christ has loved us (CCC 1967-1974).

Applying the moral law to daily life requires a relationship with God through Christ and the guidance of the Holy Spirit, which frees the believer from the law of sin (Romans 8:2, CCC 405). Moral conduct is the practical expression of this faith, love, and hope. Central to this application is the moral conscience, a judgment of reason and a sanctuary where one is alone with God (CCC 1776-1778). To determine the morality of a human act, one must evaluate the object chosen, the intention, and the circumstances (CCC 1750, 1757). This process is supported by the theological virtues of faith, hope, and charity (CCC 1812-1813), as well as the human virtues of prudence, justice, fortitude, and temperance (CCC 1805). Furthermore, the faithful are guided by the authoritative teaching of the Magisterium to ensure their conscience is well-formed and aligned with divine law (CCC 2039).