To be holy means to be set apart, to be consecrated unto the Lord. As Christians, we are already holy for by His Spirit and His word we have been set apart, sanctified unto God. The injunction to be holy, even as He who has called us is holy therefore refers to us living out or giving expression to that which we already are, i.e. living lives that are consecrated and set apart unto God.
In 2 Cor. 7: 1, 2 Paul, the apostle enjoins the Corinthian Christians saying, “Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God.”
In other words, incentive or reason why we should keep ourselves pure or cleanse ourselves from all filthiness of the flesh and spirit is because of the promises of God for he says, “Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit.” It is like he is saying, “dearly beloved, in light of these promises keep yourselves pure. These promises of which he speaks are the promises referred to in Ch. 6:16-18 i.e. the promises of the indwelling of the Holy Spirit already fulfilled (which is that God would dwell in us and by which he means that we are the temple of God), the adoption as sons (that He would receive us and would be a Father to us and we would be His sons and daughters), and citizenship and future habitation in the New Jerusalem, the city of the living God (which is that God would walk in us, that He would be our God and we would be His people; Rev. 21:3).
Therefore, as Christians, if we are going to keep his injunction to cleanse ourselves the key would be keeping constantly in our minds these promises. That is, the key to success is constantly reminding ourselves of these promises.