His Law:  

God is a sovereign King but he does not rule arbitrarily. He Rules by law. His first laws to man were given orally (in the Garden of Eden as recorded in the Book of Genesis) and later on they were written down by his own finger on tables of stone. When he called the children of Israel out of Egypt he expanded his law and they were communicated via Moses these later were written down.

Exodus 31:18

And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God.

God gave his law to Moses at Mount Sinai to establish a covenant with the Israelites. This foundational event involved thunder, lightning, and fire, as described in the book of Exodus.

Exodus 24:12-18

And the Lord said unto Moses, Come up to me into the mount, and be there: and I will give thee tables of stone, and a law, and commandments which I have written; that thou mayest teach them. ¹³ And Moses rose up, and his minister Joshua: and Moses went up into the mount of God. ¹⁴ And he said unto the elders, Tarry ye here for us, until we come again unto you: and, behold, Aaron and Hur are with you: if any man have any matters to do, let him come unto them. ¹⁵ And Moses went up into the mount, and a cloud covered the mount. ¹⁶ And the glory of the Lord abode upon mount Sinai, and the cloud covered it six days: and the seventh day he called unto Moses out of the midst of the cloud. ¹⁷ And the sight of the glory of the Lord was like devouring fire on the top of the mount in the eyes of the children of Israel. ¹⁸ And Moses went into the midst of the cloud, and gat him up into the mount: and Moses was in the mount forty days and forty nights.

God's law May refer to many different things, as follows:

1. Generically as the old Covenant or the Old Testament.
2. More specifically as the pentateuch -- that is the first five books of the Old Testament.
3. Pointedly as the Ten Commandments.

4. Collectively as the Great Commandment.

The Ten Commandments

Exodus 20:1-17

[1] And God spake all these words, saying,
[2] I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
[3] Thou shalt have no other gods before me.
[4] Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth:
[5] Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;
[6] And shewing mercy unto thousands of them that love me, and keep my commandments.
[7] Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.
[8] Remember the sabbath day, to keep it holy.
[9] Six days shalt thou labour, and do all thy work:
[10] But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
[11] For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
[12] Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee.
[13] Thou shalt not kill.
[14] Thou shalt not commit adultery.
[15] Thou shalt not steal.
[16] Thou shalt not bear false witness against thy neighbour.
[17] Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's.

The Great Commandment

Matthew 22:35-40

[35] Then one of them, which was a lawyer, asked him a question, tempting him, and saying,
[36] Master, which is the great commandment in the law?
[37] Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.
[38] This is the first and great commandment.
[39] And the second is like unto it, Thou shalt love thy neighbour as thyself.
[40] On these two commandments hang all the law and the prophets.

To love God with all our hearts is the great commandment.  We do this by obeying him and loving mankind -  created in His image.

Fulfilled in One Word

Galatians 5:13-14

[13] For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another.
[14] For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.