"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 1:15,16
God is holy.
God is holy because of who He is and not because of what He does. God is holy because He is like no other god. He is beyond compare!
"'To whom will you compare me? Who is my equal?' asks the Holy One" Isaiah 40:25
It is the "otherness" of God that defines His holiness. He is completely (wholly) good, pure, loving, righteous, just, light, merciful, gracious, etc, because that is His holy nature!
God makes holy.
In the Old Testament we read about how God brings the people of Israel out of Egypt to set them apart from all other nations as His people - a people belonging to God and set apart for Him. God thus makes His people holy by setting them apart. That which is set apart for God is sanctified - made holy.
For example, when the priests were ordained in the OT, they were set apart with blood and oil (see Leviticus 8). Likewise the believer is set apart and made holy by the blood of Jesus and the oil of the Holy Spirit.
Through faith in Jesus, the believer is sanctified by His blood and by the gift of the Holy Spirit! 1 Peter 1:1,2
The believer is therefore holy! He is sanctified. He cannot become holy in his own effort. It is God who sets the believer apart and makes Him holy. Holiness cannot be attained!
The New Testament calls God's people, saints because they have been sanctified and set apart for God. Believers cannot become saints by their own effort. They are made saints by the mercy and grace of God! See Eph. 1:1,15
"This sainthood is not an attainment, it is a state into which God in grace calls men.." W.E. Vine
You are holy!
As a believer, you are holy by nature, regardless of whether you sin or not! Your obedience or your disobedience does not change the fact that God has set you apart for Himself. As God's holy people we are called to live holy lives - lives that express our separateness for God. Paul writes,
"To the church of God in Corinth, to those sanctified in Christ Jesus and called to be holy.." 1 Cor. 1:2
The believer is called to live a holy life because he is holy and not in order to become holy!
Made holy to reveal God's holiness
God's purpose in setting apart a people for Himself, by bringing His people out of darkness, is to reveal Himself through them as the only true God - the Holy One!
"..you are a chosen people, a royal priesthood, a holy nation, a people belonging to God, that you may declare the praises of Him who called you out of darkness into his wonderful light. Once you were not a people, but now you are the people of God; once you had not received mercy, but now you have received mercy." 1 Peter 2:9,10
"Then the nations will know that I am the Lord, declares the Sovereign Lord, when I show myself holy through you before their eyes." Ezekiel 36:23
Called to holiness.
The believer is called to live a holy life because God is holy. The believer's holiness is expressed by his separateness from the world, the flesh and the devil. As God's holy people, we are in this world but not of this world.
"My prayer is not that you take them out of the world but that you protect them from the evil one. They are not of the world, even as I am not of it. Sanctify them by the truth; your word is truth. As you sent me into the world, I have sent them into the world. For them I sanctify myself, that they too may be truly sanctified." John 17:15-19
Through the believer's union with Christ, he is made holy so that he will reveal the One to whom he is joined - the Holy One (see 1John 4:15).
The way we live holy lives, as God's holy people, is to be like Jesus (1 John 2:6)! We reveal God's otherness as we reveal His nature - His love, mercy, kindness, patience, goodness, grace, forgiveness, etc.
As God's holy people "let us purify ourselves from everything that contaminates body and spirit, perfecting holiness out of reverence for God!" 2 Cor. 7:1
Saints, be holy because God is holy and you are holy!