In Leviticus 11, God is giving instructions for clean and unclean food. In verse 44, God sums everything up..."consecrate yourselves and be holy, for I am holy."
As Christians, we like to-do lists. It makes everything clear and easy to define. What God says is slightly different than what we want it to say. After giving a big list of what is and isn't clean, the next part doesn't say that this will make you holy. In essence, it is telling his people how to consecrate themselves so that they can be on their way to holiness. God gives two different commands, not different versions of the same command. The first step is to be consecrated, to set ourselves apart, to remove the things that keep us from him. Only then can we begin to become holy. Holy isn't something to do, it's something to be. It's much easier to do something that to be something. To do something just means I have to put forth some effort. To be something requires that I change who I am.
That doesn't mean that our Christian walk should be marked by inactivity since we're so busy "being" holy. There are two separate things here and we need to be careful to tend to both. Just as inactive holiness is useless, so is activity that isn't marked by holiness. Our activities do not make us holy, or else God would have told the Israelites to do those things and they would have achieved holiness. At the same time, God gave them instructions anyway. If holiness isn't something that can be "done", then what's with the list? He was showing them the necessary steps to prepare themselves.
Just as holiness isn't something that we can do, we can never reach that place without a cost. I keep hearing this idea over and over in different places and different formats. In a very real sense, my salvation is a free gift. But that doesn't mean that it is without cost. To be holy demands that a price be paid. If I am not becoming holy, I am ignoring God's explicit instruction. If I am one of his children, then verse 44 applies directly to me. I am mandated to consecrate myself so that I can become holy. Why should I consecrate myself? Because it is the path to holiness, a necessary path to take. Why should I become holy? Because my Father is and he wants me to be more like him.