When God instructs Moses to "be holy for I am Holy," it is the very first expression of the term "holy" (Ex 3:5).

Through the Mosaic legislation that takes place after the Exodus, God teaches the nation of Israel the meaning of "holy." It is a term that places an emphasis on God, because it calls to attention the condition something must be in when encountering the realm of God. To be holy, something must be set apart or separated from the profane or unclean before it can come into the presence of His glory; it applies to time, space, objects and people.

The imperative "be holy for I am holy" has a special significance, because it comes after God reveals that the nation of Israel will be His own possession and intended to be devoted to Himself:

"'Now then, if you will indeed obey My voice and keep My covenant, then you shall be My own possession among all the peoples, for all the earth is Mine; and you shall be to Me a kingdom of priests and a holy nation.' These are the words that you shall speak to the sons of Israel." (Ex 19:5-6)

Thus, the nation of Israel was obligated to be holy. In making the conditional Mosaic Covenant, the nation of Israel learned what sin was and the concept of being holy. Holiness formed the basis of ethical behavior, because it determined what was morally good or evil.

While the New Testament rarely mentions that God is holy, the term "holy" retains its Old Testament meaning of setting apart from the profane as a basis of belonging to God.

It is in Jesus that a Believer is sanctified and made holy.

It is the indwelling of the Holy Spirit that guarantees one's entrance into the kingdom of God and inheritance as God's adopted (Eph 1:13-14; 2 Cor 1:21-22; 5:5).

The concept of the holy was erroneously understood in the context of terrifying awesome power. In fact, it is when the Divine comes in contact with sin or the unconsecrated that awesome power is terrifying!

To learn more, see:

What did sanctification mean in the Old Testament?

"How does God sanctify human beings?