Note: This post is a summary of a section of "The threefold division of the law," by Jonathan F. Bayes, which we highly recommend.
In "Chapter 20. Of Civil Government" of The Institutes, Calvin writes:
We must attend to the well-known division which distributes the whole law of God, as promulgated by Moses, into the moral, the ceremonial, and the judicial law.
Francis Turretin was one of Calvin's successors in Geneva. He writes:
The law given by Moses is usually distinguished into three species: moral (treating of morals or of perpetual duties towards God and our neighbour); ceremonial (of the ceremonies or rites about the sacred things to be observed under the Old Testament); and civil (constituting the civil government of the Israelite people).
In "Chapter 19. Of the Law of God" of the 1689 London Baptist Confession (which is extremely similar to what the Westminster Confession of Faith says on this subject), we read:
1. God gave to Adam a law of universal obedience written in his heart, and a particular precept of not eating the fruit of the tree of knowledge of good and evil; by which he bound him and all his posterity to personal, entire, exact, and perpetual obedience; promised life upon the fulfilling, and threatened death upon the breach of it, and endued him with power and ability to keep it. ( Genesis 1:27; Ecclesiastes 7:29; Romans 10:5; Galatians 3:10, 12
2. The same law that was first written in the heart of man continued to be a perfect rule of righteousness after the fall, and was delivered by God upon Mount Sinai, in ten commandments, and written in two tables, the four first containing our duty towards God, and the other six, our duty to man. ( Romans 2:14, 15; Deuteronomy 10:4
3. Besides this law, commonly called moral, God was pleased to give to the people of Israel ceremonial laws, containing several typical ordinances, partly of worship, prefiguring Christ, his graces, actions, sufferings, and benefits; and partly holding forth divers instructions of moral duties, all which ceremonial laws being appointed only to the time of reformation, are, by Jesus Christ the true Messiah and only law-giver, who was furnished with power from the Father for that end abrogated and taken away. ( Hebrews 10:1; Colossians 2:17; 1 Corinthians 5:7; Colossians 2:14, 16, 17; Ephesians 2:14, 16
4. To them also he gave sundry judicial laws, which expired together with the state of that people, not obliging any now by virtue of that institution; their general equity only being of moral use. ( 1 Corinthians 9:8-10
5. The moral law doth for ever bind all, as well justified persons as others, to the obedience thereof, and that not only in regard of the matter contained in it, but also in respect of the authority of God the Creator, who gave it; neither doth Christ in the Gospel any way dissolve, but much strengthen this obligation. ( Romans 13:8-10; James 2:8, 10-12; James 2:10, 11; Matthew 5:17-19; Romans 3:31