What is mean to be holy? The basic meaning is to be set apart, to be different in a way that glorifies God. It can also be considered in the sense of being un-defiled, clean, pure. There are two primary word used for holy.
H6944 Hebrew
qôdesh ko'-desh
From H6942; a sacred place or thing; rarely abstractly sanctity: - consecrated (thing), dedicated (thing), hallowed (thing), holiness, (X most) holy (X day, portion, thing), saint, sanctuary.
In the Greek
hagion
[H2282]; sacred (physically pure, morally blameless or religious, ceremonially consecrated): - (most) holy (one, thing), saint.
God is the supreme example of being Holy. There is none like Him nor ever will be. He is absolutely pure, perfect, without blemish, and utterly righteous. It is His unique divine nature and being that provides us with this understanding. The challenge God gives to all true believers is to be holy as He alone is holy.
Leviticus 19:1-2 And Jehovah spoke to Moses, saying, Speak to all the congregation of the sons of Israel and say to them, You shall be holy, for I, Jehovah your God, am holy.
Leviticus 20:26 And you shall be holy to Me. For I, Jehovah, am holy, and have severed you from the nations, so that you should be Mine.
1 Peter 1:13-16 Therefore girding up the loins of your mind, being sober, perfectly hope for the grace being brought to you at the revelation of Jesus Christ, as obedient children, not fashioning yourselves according to the former lusts in your ignorance, but according to the Holy One who has called you, you also become holy in all conduct, because it is written, "Be holy, for I am holy."
Holiness is not a subject often preached or practiced in most churches today. We have much to learn and unlearn.