But as He who called you is holy, you also be holy in all your conduct, because it is written, "Be holy for I am holy."
God calls us to holiness, or absolute purity. The first thing that comes to my mind when I consider this demand on my life is my total inability to be holy, because I know that I am not perfect; I know that I am a sinner.
However, recognizing my sinfulness should point me to Jesus Christ and the life that He is for sinners. "If you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved." (Rom. 10:9). The verse above from Peter is as much a promise that God will make us holy as it is a command to be holy. Those who trust in Christ have His holiness and righteousness covering their filthy rags of sin.
Something amazing about the gospel is that we are presenly united to Christ. This means, as Ephesians 2:6 says, we were raised up together, and made to sit together in the heavenly places in Christ Jesus. As those who are in Christ, we are already holy. This is why we must be holy in all our thoughts, words, and deeds. Paul later says in Ephesians, "For you were once darkness, but now you are light in the Lord. Walk as children of light (for the fruit of the Spirit is in all goodness, righteousness, and truth), finding out what is acceptable to the Lord." (5:8-10). His emphasis here is that since we are light we must act like it.
May we prize God so highly that our desires are always to glorify Him. As He glorifies himself in us He will give us the grace we need to walk in holiness.