Since the children of Israel were not spiritually ready for the Melchizedek Priesthood and its ordinances, the Lord prepared them under the Aaronic or Levitical Priesthood and gave them the Law of Moses (See D&C 84:23-27).
The book of Leviticus is basically a 'handbook' of how to function in the Levitical Priesthood and how to administer the sacrificial ordinances of the law of Moses. It contains detailed instructions concerning different kinds of sacrifices and religious practices which may seem strange to those in the modern world. Looking past those practices however, we can learn the hidden truths about sin, repentance, forgiveness, holiness, and the Atonement of Jesus Christ.
The most important theme in the book of Leviticus is the process of becoming holy, and the book is sometimes called "The Holiness Code." It is interesting to note that the word holy (or a word that comes from the same Hebrew root, such as the English "sanctify" which is translated from the verb "to make holy" in Hebrew) is listed over 150 times in Leviticus. Leviticus expresses what one must do ritually in order to become holy: such as dietary, health, and clothing restrictions. These restrictions have been interpreted as being symbolic of becoming clean and free from the effects of sin and becoming justified before God. Holiness is more than being clean; it includes the process of becoming godlike in character. Therefore Leviticus commands: "ye shall be holy; for I am holy" (Leviticus 11:44, compare 1 Peter 1:16). Thus, one facet of holiness and sanctification is the process of becoming like God. Another view of holiness based upon its usage in the ritual requirements of Leviticus is that holiness involves being set apart, separate, and dedicated for special service to God.
The organization of the book states a pattern of spiritual development for all: Chapters 1-7 describes various kinds of sacrifices that the people were to make. These sacrifices embodied the Savior and his atoning sacrifice; Chapters 8-10 explains the requirements placed upon the priests for them to be worthy to perform the sacrifices; Chapters 11-15 explains various laws concerning cleanliness and uncleanliness, stressing its importance. These laws demonstrated the need to be clean personally, as families, and a people; Chapter 16 is the spiritual culmination of all the laws of cleanliness. It gives directives concerning the great and refining sacrifice offered each year on the Day of Atonement.