Catechism of the Catholic Church
2070 The Ten Commandments belong to God's revelation. At the same time they teach us the true humanity of man. They bring to light the essential duties, and therefore, indirectly, the fundamental rights inherent in the nature of the human person. The Decalogue contains a privileged expression of the natural law:
From the beginning, God had implanted in the heart of man the precepts of the natural law. Then he was content to remind him of them. This was the Decalogue. 31
2071 The commandments of the Decalogue, although accessible to reason alone, have been revealed. To attain a complete and certain understanding of the requirements of the natural law, sinful humanity needed this revelation:
A full explanation of the commandments of the Decalogue became necessary in the state of sin because the light of reason was obscured and the will had gone astray. 32
We know God's commandments through the divine revelation proposed to us in the Church, and through the voice of moral conscience.
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.