In the Greek, the conditional clause is stated first before the exhortation (see Revised Standard Version), but the Good News Translation and most modern translations have reversed the order, to conform to better English style.
Instead translates “but,” which is a very strong adversative in the Greek (compare Biblia Dios Habla Hoy “on the contrary”). The model of obedience must not be their former way of life, but the very character of God himself. The strong adversative conjunction in Greek may be expressed as “on the contrary” or “instead of letting your desires shape your life.”
Be holy in all that you do is literally “be holy in every conduct,” which includes all actions, both in the personal and social spheres. The concept of “holiness” is difficult both to analyze and to translate. The following elements may be mentioned as included in the concept: (1) To be holy is to be separated from worldly relations and to be consecrated and dedicated to God. In this sense, “holiness” is primarily a relationship term: people are holy because they are chosen by God to be his people, and they now belong to him. (2) Holiness may also be understood in an ethical sense: to be holy is to be conscious of belonging to God and to act out this relationship in obedience to God’s will. A very important aspect of this ethical understanding of holiness is moral integrity, which includes hatred for everything evil and sinful, and complete dedication to everything that is good and acceptable to God. (3) Holiness, when applied to God, emphasizes God’s moral perfection, his perfect goodness, and consequently his dislike of anything evil. It is this character of God which becomes the basis for the “holiness” of Christians, who are separated from sin and evil, and are instead consecrated to God, in order to do God’s will and be obedient to him.
In the light of the above analysis, it is probably much better to understand “holiness” ethically. To be “holy in every conduct” is to hate evil, to refrain from every evil deed, and to obey God’s will in everything.
One of the most difficult terms to translate adequately is holy. Some translators have endeavored to use a term which means simply “separated,” but it is usually necessary to indicate from what one has been separated or to what one has been separated. One might, for example, translate be holy in all that you do as “in everything that you do, separate yourself from that which is evil” or “… that which is sinful.” In certain contexts one may render holy as “separated for God” or “dedicated to God,” but in this context it would be difficult to employ such an expression since the relationship between holy in the first part of verse 15 and holy in the second part would be difficult to maintain, for one could not speak of God as being “separated for himself.” It is unfortunate that in a number of languages one can only speak of “holy” from a negative standpoint, that is to say, “removed from evil.”
Just as God who called you is holy is literally “according to the holiness of the one who called you.” Called has reference probably to the time that the readers of the letter were converted to Christianity. In the New Testament, the role of calling people to believe in Christ and to become members of the Christian community is assigned primarily to God, and the Good News Translation has made this information explicit. For holy, see discussion above.
Quoted with permission from Arichea, Daniel C. and Nida, Eugene A. A Handbook on The First Letter from Peter. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .