What is The Law of God?
God gave to Adam a law of universal obedience written in his heart, and a specific precept not to eat the fruit of the tree of knowledge of good and evil. By this He bound him and all his descendants to personal, total, exact, and perpetual obedience. God promised life on fulfilling it, and threatened death on breaching it, and He endued him with power and ability to keep it.
The same law that was first written in the human heart continued to be a perfect rule of righteousness after the fall. It was delivered by God upon Mount Sinai in Ten Commandments (written in two tables) the first four containing our duty towards God, and the other six our duty to our fellow beings.
Besides this law, commonly called the moral law, God was pleased to give the people of Israel ceremonial laws containing several typical ordinances. These were partly concerning worship, and in them Christ was prefigured—His graces, actions, sufferings, and benefits. They also gave instructions about various moral duties. All of these ceremonial laws were appointed only until the time of the New Testament, when Jesus Christ abrogated them and took them away, for He was the true Messiah and only law-giver, and was empowered to do this by the Father.
To the people of Israel He also gave various judicial laws which lapsed when they ceased as a nation. These are not binding on anyone now by virtue of their being part of the laws of that nation, but their principles of equity continue to be applicable in modern times.
Obedience to the moral law remains forever binding on all, both justified persons and others, both in regard to the content of the law, and also to the authority of God the Creator who gave the law.Nor does Christ in any way dissolve this law in the Gospel, on the contrary, He strengthens our obligation [to obey the moral law].
Although true believers are not under the law as a covenant of works to be justified or condemned by it, yet it is of great use to them as well as to others, because as a rule of life it informs them of the will of God and their duty, and directs and binds them to walk accordingly.
It also exposes the sinful defilement of their natures, hearts and lives, and as they use it to examine themselves, they come to greater conviction of sin, humiliation for sin, and hatred against sin. They also gain a clearer sight of their need of Christ, and the perfection of His obedience [to the law].
Similarly, it is of use to the regenerate to restrain their corruption in that it forbids sin. The threatening of the law serves to show what even their sins deserve, and what troubles they may expect in this life because of their sins, even though they are freed from the curse and undiminished rigors of the law.
The promises of the law also show believers God's approval of obedience, and what blessings they may expect when the law is kept, although these blessings are not due to them through the law as a covenant of works. If someone does good and refrains from evil simply because the law encourages the former and deters from the latter that is not evidence of one's being under the law and not under grace.
These uses of the law are not contrary to the grace of the Gospel, but are entirely in line with it, for the Spirit of Christ subdues and enables the human will to do freely and cheerfully what the will of God revealed in the law requires to be done.
 For the Fourth Commandment: Genesis 2:3; Exodus 16; Genesis 7:4; 8:10,12. For the Fifth Commandment: Genesis 37:10. For the Sixth Commandment: Genesis 4:3-15. For the Seventh Commandment: Genesis 12:17. For the Eighth Commandment: Genesis 31:30; 44:8. For the Ninth Commandment: Genesis 27:12. For the Tenth Commandment: Genesis 6:2; 13:10-11