The word ‘holy’ is a translation of the Hebrew word qadosh and the Greek word hagios. The simplest meaning of this word is ‘separated’ or ‘set apart’. The Scriptures tell us that God is holy, that is, He is set apart from us. He is in a completely different category to human beings. The word ‘holy’ also has a sense of purity, of complete moral goodness. So to say that God is holy is to say that He is in a completely different category in terms of His moral perfection and His absolute purity.
The Bible also says that because God is holy, so too should His people be holy (Leviticus 11:44-45, 1 Peter 1:15-16). They should be set apart, different from the world around them - they should be morally pure. However, it is not possible for us to become holy by ourselves, as even our good deeds are like filthy rags when compared to God’s holiness (Isaiah 64:6). We need Him to make us holy - to sanctify us (the word ‘sanctify’ is derived from the same Greek word hagios). The good news is that in Christ we have been made holy by His sacrifice, and we continue being made holy by the power of the Holy Spirit at work in us (Hebrews 10:10-18). Holding on to this truth, we are to strive for holiness, putting aside all evil behaviour, and purifying ourselves, as is fitting for a Holy God (1 Peter 1:13-25).