1 Peter 1:13-16
13 Therefore, with minds that are alert and fully sober, set your hope on the grace to be brought to you when Jesus Christ is revealed at his coming. 14 As obedient children, do not conform to the evil desires you had when you lived in ignorance. 15 But just as he who called you is holy, so be holy in all you do; 16 for it is written: “Be holy, because I am holy.”
Peter starts his letter to the believers reminding of the inheritance they have by becoming children of God. Today, he moves to how they should respond in light of the gift of grace they have received. He urges them to be holy as He who has called them is holy. A lot of times we misunderstand the notion of "holiness". We begin with grace, then think holiness is something we achieve in our own. There is an inherent problem in this. It never works.
Since we are a new creation in Christ, becoming holy is about living in the grace we have been given. As we understand the gift of grace we have been given, we are motivated toward obedience not to gain acceptance, but because we already have it. The problem with trying to earn our holiness, is that in and of ourselves we are not holy. But because God is holy and we have entered into a relationship by grace, we become holy because of what he has done for us.
But this doesn't mean the process of sanctification is a passive process on our part. Peter uses the word "obedience", and says "be holy" showing that we have a role to play as well. He also says do not conform to the evil desires you once lived in ignorance. But now they no better! Part of the process of becoming holy is consciously turning away from an old way of life. An old way of life where we were being conformed by the world, instead being transformed by Christ.
Trying to act or be holy in our own power never works. But as we are obedient to our Heavenly Father in His power we can become more like him who is holy.