Because of sin, not everyone clearly perceives the natural law (417). For this reason, St. Augustine said, God “wrote on the tables of the law what men did not read in their hearts.” The Ten Commandments summarize the moral teaching of the Old Testament, also known as “the Old Law.” In this context, “old” does not mean outmoded, useless, or no longer true. Rather, it means that the moral teaching of the Old Testament was the first stage of the revealed law, which in turn was completed and fulfilled by the Gospel.
The Ten Commandments express moral truths known naturally by reason, thus verifying the natural law. In laying the foundations for the human vocation to love God and neighbor, they are “a privileged expression of the natural law” (Catechism, 2070; Compendium, 418). In teaching unchanging moral truth, they are also a “tutor” that prepared the way for the Gospel. The Old Law, however, remains imperfect. While it teaches moral truth, it does not provide the strength of the Holy Spirit (419).
“The New Law,” which can be found in the New Testament, especially in the Beatitudes, was both proclaimed and fulfilled by Christ (420-421). Here the word “new” does not indicate a rupture with the Old Law. Jesus said, “Do not think I have come to abolish the law or the prophets. I have come not to abolish but fulfill” (Mt 5:17). The Gospel is new because it originated in the person of Christ, the incarnate Son of God, whose teaching, death and resurrection fully revealed the Father’s love. Through the Holy Spirit, believers share in Christ’s risen life, are drawn into intimacy with the Father and are enabled to love others as they are loved by Christ (420).
Indeed, Christ came to “justify” us in the power of the Holy Spirit. Justification is God’s mercy at work to make us holy by granting us remission of our sins and a share in his goodness. We are justified by the grace of the Holy Spirit, which Christ won for us by his death and resurrection. God freely gives us grace, a share in his divine life that enables us to respond to his love and to live in his friendship. This is called “sanctifying” or “deifying” grace because it makes us participants in the life of the Trinity. This gift is supernatural because it cannot be learned or earned; it can only be received from God. Sanctifying grace is “habitual” because we are to remain in the state of grace throughout our lives (423).
In addition, there are “actual” graces that help us do God’s will in the various circumstances of our lives. Each sacrament also confers a special grace on us (424). Ultimately, God’s grace does not infringe on our human freedom but frees us to fulfill our deepest yearnings to share his life and love (425).