Instead, as he who called you is holy, be holy yourselves in all your conduct; for it is written, “You shall be holy, for I am holy.”
This appears to be a wisdom saying drawn from the Old Testament. It occurs four times in the book of Leviticus (11:44, 11:45, 19:2, 20:7). Each time, it is in the context of specific behavior, being holy according to the dietary rules (ch. 11), being holy according to reverence paid to parents (ch. 19), and being holy in the context of not going to strange religious practices (ch 20). I must be from a dark side parental mindset when I read these passages. Moses has to reinforce these rules with calls to be holy because these were precisely the behaviors that the people were most disobedient about.
And God chose not to destroy them.
That may be the most miraculous part of the Mosaic law, the fact that God did not destroy those people. They gave Him a run for His money at every turn. He got so angry that he threatened to destroy them once and start again with the family of Moses. No lie.
“You shall be holy, for I am holy.” It is not a statement of requirement. It is a statement of promise. Even those stiff necked people were going to be holy , because God is Holy. It is the promise that Peter is drawing upon when he said “For it is written…” In other words, this is particularly important for the readers to track in on.
Holiness is a virtue that comes from God, from obedience to God, from seeking to do God’s will, from doing as Jesus did. It is the setup Peter begins his letter with. It is the call he issues to his readers now. It comes from the transformation of the worst behaviors to those acceptable, even approved of, even adored by our God in heaven. Such is the practice of faith.
I have yet to meet someone who truly feels himself or herself truly worthy of God, they would not consider themselves holy by any stretch of the imagination. But the promise Jesus gives to us through Peter’s words are that we can be holy, and we shall be holy, because God already is.