The command, “Be ye Holy” is not humanly possible. Therefore when someone is born again, GOD cleanses the spirit and soul and IMPUTES the righteousness of Christ (The Holy One of Israel) to the new born. In that way is fulfilled the command “Be ye Holy”.
He said, "Be holy, just as I also am holy.". You're right, it is IMPOSSIBLE for anyone to be as holy as God unless they have the righteousness of Jesus Christ imputed to them. We are made as righteous and holy as God through faith in Christ because of the grace of God. And it is this same grace that sustains us and enables us to walk in holiness. The just shall live by faith.