Holy isn't a common or well understood word nowadays. Even Christians often confuse its meaning with sinless or perfect. But God would never command us to do something that is impossible for us, and several times in Leviticus (Lev 11: 44, for example) He commands His people to, "Be holy as I am holy."
The Hebrew word is qadosh and it means to be set apart, to be morally pure, to be different (which links it to yesterday's understanding of God being unique, different from other gods.)
But of course, God is sinless and perfect, so that's why Bible characters who have a vision of His holiness (as in Isaiah chapter 6) find it almost overwhelming. Nehemiah and David both use the word awe-inspiring or awesome. That's with the original meaning of awesome; older translations use the word terrible or fearsome. (Nehemiah 1: 5, Psalm 68: 35)
God, the Holy One, can't stand sin; He can't stand the harm it does to people and to the world. So He commands us to be holy - to be morally pure and to deal radically with sin. When we don't live holy lives, we bring shame on God's holy name. (See Ezekiel 36: 20 - 22)
Of course, we humans can never attain God's level of sinless perfection. That's why, all through the Bible, He provides ways of dealing with our sin (Psalm 103: 3 - 12; 1 John 1: 9) so that we can continue to know Him and reflect His holiness.
God's perfect holiness places Him high above the level of mankind. In Isaiah 57: 15, He tells us that His name is Qadosh, that He lives in eternity; He lives in a "high and holy place"... but He also lives with those whose hearts are humble and repentant. His holiness, His hatred for evil and wickedness push Him to eradicate all that would prevent us from being holy too. The only condition is that we want it too, and that we are quick to confess our sin with, "humble and contrite hearts."
If our realisation of our sin hasn't left us "crushed" or "contrite," we may be slow to embrace our need for cleansing. Perhaps we need to pray for a revelation of God's overwhelming holiness. Only then will we have the same humble and repentant response that Isaiah had.
