God describes himself as “holy.” But when researching the meaning of “holy,” I found this quote in the 1952 edition of the Dictionary of the Bible edited by James Hastings, D.D. on the word “holiness”:

The primary meaning seems at present indiscoverable, some making it to be that of 'separation' or 'cutting off,' others connecting with chadash 'new', and the Assyr. Quddushu 'pure,' 'bright'; but neither brings conclusive evidence. In actual use the word is always a religious term, being, when applied to deity, almost equivalent to 'divine,' and meaning, when used of persons or things, 'set apart from common use for divine use.'

God told Moses: “Speak to the entire assembly of Israel and say to them: 'Be holy because I, the LORD your God, am holy'” (Leviticus 19:2).

The apostle Peter wrote: “As obedient children do not conform to the evil desires you had when you lived in ignorance. But just as he who called you is holy so be holy in all you do; for it is written: “Be holy; because I am holy” (1 Peter 1:14-15).

NIV (The NIV Study Bible, New International Version, Copyright 1985 by The Zondervan Corporation) study note explains this exhortation as—“set apart from sin and impurity, and set apart to God” and “to strive for moral purity.” The context supports this explanation.

God stated he created man in his own image (Genesis 1:26-27). From the beginning mankind was meant to become like God. One description of God's likeness is “holy.” What does it mean to be “holy?”

We find a few clues in the book of Leviticus which describes how the nation of Israel was to be holy (Leviticus 19:1-2). Sacrifices were to be of the best and blemish free; animals were to be firstborn. They were holy sacrifices, dedicated to God. They pointed forward to Jesus who became that perfect sacrifice so we could have our blemishes and stains removed (1 Peter 1:13-19; Hebrews 9:6-28).

We also are to be “living sacrifices” dedicated to God:

Therefore, I urge you, brothers, in view of God's mercy, to offer your bodies as living sacrifices, holy and pleasing to God—this is your spiritual act of worship. Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is—his good, pleasing and perfect will.
Romans 12:1-2

We don't become holy by living a moral life—not one person except Jesus ever lived sin free. The only way to become holy is by being united with Jesus through belief in him and receiving the Holy Spirit.

I have been crucified with Christ and I no longer live, but Christ lives in me. The life I live in the body, I live by faith in the Son of God, who loved me and gave himself for me. I do not set aside the grace of God, for if righteousness could be gained through the law, Christ died for nothing!
Galatians 2:20-21

We are promised that when Jesus returns we will be like him “for we shall see him as he is. Everyone who has this hope in him purifies himself, just as he is pure (1 John 3:2-3). The way we “purify” ourselves is through Jesus who is taking away our sins (1 John 3:5).

Only God is truly holy; only in God can we become holy as he is holy.