In the Law of Moses, God commanded Israel to be holy because He is holy.
Following the flood, when the second world order began at Babel, GOD assigned angels to oversee the Gentile nations, but kept Israel for Himself to rule over as King (even though they did not yet exist at that time).
Regarding GOD’s selection of Israel, Moses wrote:
9 For the Lord’s portion became the people of Jacob;
The allotment of His inheritance is Israel.
10 “He provided for him in the desert,
In thirst of heat in a waterless place.
He encircled him; He instructed him;
He guarded him carefully as the apple of His eye.
11 As an eagle covers its nest,
Yearns after its young,
Spreading out its wings, taking them up,
Carrying them on its back,
12 So the Lord alone led them [Israel],
And there was no foreign god with them.
13 “He raised them on the strength of the land;Deuteronomy 32:9-14 OSB
He fed them on the produce of the fields;
He suckled them on honey from the rock
And oil from the solid rock,
14 Butter of oxen and milk of sheep,
With fat of lambs and goats,
Of sons of bulls and goats,
With the choicest wheat;
And you drank wine, the blood of grapes.
In the Law of Moses, GOD commanded Israel to be holy because He is holy:
1 Now the Lord spoke to Moses, saying, 2 “Speak to all the congregation of the children of Israel and say to them, ‘You shall be holy, for I the Lord your God am holy.Leviticus 19:1-2 OSB
How was Israel set apart as a nation?
By definition, this made Israel holy in contrast to the other nations. This was proper order.
We previously established that these overseeing angels eventually flipped from righteousness to do evil. This rebellion and idolatry was occurring as far back as during Abraham’s lifetime, for GOD revealed to Abraham that his seed would be strangers in a foreign land for four generations, because the wickedness of the Amorites was not yet complete (see Genesis 15:16).
Likewise, GOD said He was executing vengeance upon the gods of Egypt (see Exodus 12:12) with the final plague.
Between the time that the angels were assigned rulership of the Gentile nations at Babel and the time Israel left Egypt in the exodus, Satan had corrupted the ruling angels of at least some, if not all, of the Gentile nations, as we see with Egypt and Amon cited above.
I surmise that the reason GOD called Abraham—for the ultimate purpose of creating a people (the saved, by faith in Jesus) who would come into existence through the lineage of Israel in the person of Jesus—and created the Israelites was because all the angelic rulers had become compromised, so there were no more large groups of holy people on the earth (prior to Israel). In other words, Israel was created for the purpose of being different.
So that’s Israel’s part of the “Be holy for I am holy” equation. What about GOD’s part?
Well, we could simply say that GOD is wholly other (entirely set apart) from all creation, and that would be true. But that misses the depth of why GOD based Israel’s holiness upon His own.
Because the ruling angels—and Satan before them, as the father of evil—chose to do evil instead of remaining faithful to their assigned roles, these “gods” became un-holy, distinguishing them (negatively) from Jehovah GOD, who is (supremely!) holy (holy holy). (Amen!)
GOD is holy (set apart) from the other ruling heavenly creatures of the second world because He is righteous and they did evil, making them unholy.
This part ☝️ (GOD being holy) came first, chronologically, in example, and in preeminence.
Because GOD is holy, and because He hates sin, He commanded Israel—the nation of His people—to be holy.
Recall back to where it all began, as soon as Israel safely crossed the Red Sea and arrived at Mount Sinai, GOD’s first words to the people were:
1 Now in the third month after the children of Israel departed from the land of Egypt, on the same day, they came into the Sinai Desert. 2 For they departed from Rephidim, came into the Sinai Desert, and camped in the desert. So Israel camped there before the mountain.
3 Then Moses went up to the mountain of God, and God called to him from the mountain, saying, “Thus you shall say to the house of Jacob and tell the children of Israel: 4 ‘You have seen what I did to the Egyptians, and how I bore you on eagles’ wings and brought you to Myself. 5 Now therefore, if you will indeed obey My voice and keep My covenant, you shall be a special people to Me above all nations; for all the earth is Mine. 6 You shall be to Me a royal priesthood and a holy nation.’ These are the words you shall speak to the children of Israel.”Exodus 19:1-6 OSB
Therefore, while the holiness of Israel is based upon the fundamental holiness of GOD’s character, this should help us more deeply appreciate the reasons why GOD commanded Israel to be holy as a people.