Leviticus 20:26 (ESV) … “You shall be holy to me, for I the Lord am holy and have separated you from the peoples, that you should be mine.”
Leviticus 20: 26 calls for a renewal for the Israelites to ‘sanctify’ themselves and ‘be holy’, and so imitate the Lord their God who is holy (Lev 19:2). Furthermore, holiness is clearly identified with separation. Not only does the word ‘holy’ itself suggest separation, but God spells it out that they are to be holy because the holy God has ‘separated’ Israel from the nations to belong to him.
The phrase, ‘separated you from the peoples’ (20:26), is a repetition of the words at the end of 20:24 “I am the Lord your God, who has separated you from the peoples.”
In this way we learn very clearly that sanctification (holiness) cannot be divorced from the separation that God demands. Holiness involves separation from all that is unclean and separation to what is clean. Israel is to be separate from the immoral practices of the pagan cultures. We are called to be separate to God by keeping his statutes that display his moral characteristics.
Holiness is not something we can earn or achieve by our own efforts. It is, first of all, a state created by God which we are then called to maintain. Christians are called to live differently because God has sanctified them through his Son and Spirit. Nobody can become holy if he or she has not been set apart by God through the Spirit’s regenerating work and Christ’s atoning blood. The call to obedience is set in this context of the Lord who sanctifies. We then sanctify ourselves through obedience to God’s revealed Word.
 Eveson, P. H. (2007). The Beauty of Holiness: The Book of Leviticus Simply Explained (pp. 280–281). Darlington, England: Evangelical Press.