There are aspects of moral and ceremonial cleanness associated with the word Kaw-doshe (Holy) and aspects of being set apart, as in something that is holy unto the Lord:
Leviticus 27:30 - Every tithe of the land, whether of the seed of the
land or of the fruit of the trees, is the LORD's; it is holy to the
These aspects, when applied to people regarding anything within creation, require a reaction. We must think right, speak right, do right, etc.
When applied to Almighty God the predominant meaning is one of "otherness/separateness".
In the year that King Uzziah died I saw the Lord sitting upon a throne, high and lifted up; and the train of his robe filled the temple. Above him stood the seraphim. Each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. And one called to another and said: “Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory! - Isaiah 6:1-3
Yahweh is absolutely "other" (separate, Holy) than anything He has created and, since there is only God and that which God has made, He is singularly "other".
Holiness for God is an intrinsic attribute...He IS Holy and is unlike anything else. God's "otherness" incorporates His purity, perfection, unity, etc., in that He is unlike anything else in every and each of His attributes.
Holiness for the people of God is an upward call and an outward call. As Israel was called to be a peculiar people, so we are to be unlike the world around us...seeking worldly wealth, selfish ambition, instant gratification, retribution, etc:
After this I saw another angel coming down from heaven, having great authority, and the earth was made bright with his glory. And he called out with a mighty voice, “Fallen, fallen is Babylon the great! She has become a dwelling place for demons, a haunt for every unclean spirit, a haunt for every unclean bird, a haunt for every unclean and detestable beast. For all nations have drunk the wine of the passion of her sexual immorality, and the kings of the earth have committed immorality with her, and the merchants of the earth have grown rich from the power of her luxurious living.” Then I heard another voice from heaven saying, “Come out of her, my people, lest you take part in her sins, lest you share in her plagues; for her sins are heaped high as heaven, and God has remembered her iniquities. - Revelation 18:1-5
We are to be holy unto the Lord because He IS Holy. Separate from the rest of creation. Peculiar. In the world but not of the world. Strangers and pilgrims looking for that dwelling place whose author and builder is God.
By faith Abraham obeyed when he was called to go out to a place that he was to receive as an inheritance. And he went out, not knowing where he was going. By faith he went to live in the land of promise, as in a foreign land, living in tents with Isaac and Jacob, heirs with him of the same promise. For he was looking forward to the city that has foundations, whose designer and builder is God. - Hebrews 11:8-10
We are called to a life energized by faith not by sight. We are called to live within the Kingdom of Heaven not the kingdoms of earth. For this one must swear fealty to the King which Heaven has established; Jesus Christ, the Son of the Living God. Abraham saw His day and was glad. By that faith Abraham was born again and so must we be.
Jesus answered him, “Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.” Nicodemus said to him, “How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?” Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. - John 3:3-6
Apart from faith in the Lord Jesus Christ there is no hope of holiness because holiness is an approach, a setting apart, unto God and no one comes to the Father except through Him.