Because He is Holy
While holiness is mentioned in other places in the first few books of the Old Testament, we begin to see what it really means in the book of Leviticus.
Leviticus lays out that, "The distinctions between holy and common and clean and unclean." (Hamilton p. 259)
God has shown that He is set apart from the common things, and His command is for the people of God to be set apart as well.
We find the command to “Be holy for I am holy” three times in the book of Leviticus. Ones in chapter 11, then again in chapters 19 and 20.
Each of these places help point out to us what the holiness of God means in relation to us, and how we are to live holy because He is holy.
The first reference to the holiness starts, “ For I am the Lord your God: ye shall therefore sanctify yourselves, and ye shall be holy; for I am holy: neither shall ye defile yourselves with any manner of creeping thing that creepeth upon the earth.” (Leviticus 11:44)
The reference to eating is not by accident when we think about our conduct during these times. It is during mealtimes that the children of Israel would have been alone or with family, these were private times. Hamilton helps explain this by showing “It is a holiness put into practice among family members and behind closed doors. It is a holiness faithfully lived out even when no one is watching.” (Hamilton p.263)
We are called to be holy when no one is watching, because He is holy.
Secondly, Leviticus 19 shows us that holiness should extend to one showing “reverence his mother and his father” (Leviticus 19:2-3). The remainder of this chapter show “obedience to parents and God; respect for the elderly; meeting the physical needs of the poor; telling the truth; rising above the temptations of injustice.” (Hamilton p. 288)
We are called to live holy before others, because He is holy.
Lastly, we can see the holiness of God in the command that "you shall consecrate yourselves therefore” (Leviticus 20:7). This reference was in mentioning the worship of other gods.
We are to live holy in of consecration to God, because He is holy.