Be holy for I am holy

God's calling to "be holy for I am holy" sounds impossible and over-the-top. We may think, "how can I ever be as good, righteous and holy as God!?" 

But might this be a misunderstanding of this command? Note the verse does not say do holy as I do holy. It is addressing our state of being; who we are, not what we do. This is not a focus on rightness of conduct i.e. "living right," though it certainly includes and results in right behavior. 

This involves singleness of focus-devotion i.e. It is to value (treasure) God above all things, having our hearts wholly devoted to him in the same way he is to himself. It's having the ultimate affection of our hearts captured by him and by nothing else. We value (worship) Him above everything. Our devotion to him is distinct or separate (holy) and greater than any other devotion because He is distinctly and immensely more valuable and greater than everything.  

We could even argue that a fair translation would be, "be wholly devoted to me and my glory in the same way I am wholly devoted to it." 

Or as Christ said if our eye is healthy -- not blurry but clear and focused -- then we are full of light (truth) i.e. right conduct/holiness is the fruit of clearly (truly) seeing God in all his glory and valuing (treasuring) Him accordingly. 

How and when does this occur?

It occurs only when we see the true value/glory of God. The more clearly we see him in all his beauty and glory, the greater our treasuring of Him and affections for Him, and the more we are drawn to him and the harder we pursue him.

So our focus is not on right behavior/conduct, it's about right seeing and right valuing; seeing God clearly (with singleness of focus) and truly as he is...as all-loving, wise, good and glorious and most worthy of our highest praise, adoration, devotion and love. We will never do so if our gaze/affection is more set on something other than him (including right conduct) i.e. We will never be holy as he is holy by "trying" to be holy.

Summary

Holiness is not being perfect in conduct (though it certainly is the cause of our behavior becoming increasingly more perfect) but being perfected in devotion which is the fruit of an increasingly clearer view of his great glory/worth resulting in our treasuring Him above everything else. As our view of God in all his love and glory becomes clearer, our value of God grows as well as our devotion and faithfulness to Him; our behavior becomes more righteous and increasingly reflects the greatness of his worth. 

Mat 6:21  For where your treasure is (that which you value/treasure most and have the greatest affection for), there your heart (longings/desires) will be also. 22 "The eye is the lamp of the body. So, if your eye (focus) is healthy (clear and on the right thing i.e. that which has true and lasting value), your whole body (everything else about you; your actions, conduct etc) will be full of light, 23 but if your eye (focus) is bad (on the wrong thing), your whole body (your conduct) will be full of darkness. If then the light (your focus) in you is darkness, how great is the darkness! 

This verse clearly indicates that the body - by which our actions are carried out - moves as the result of our focus (eyes) being set on either the right or wrong thing. Actions are always the result or fruit of our focus. Our focus is always on what we value most. And what we value most is where our greatest affections and longings lie i.e. we are most affectionate about what we treasure most. In time we are shaped and eventually become more like what we adore most. Our affections shape our behavior.