When we are saved and are born again our desires change from being self-centered to God-centered. What does God require? How can we please Him? One such requirement is found in 1 Peter.
1 Peter 1:15-16 (KJV)
But as he which hath called you is holy, so be ye holy in all manner of conversation;
Because it is written, Be ye holy; for I am holy.
We are told here, that because God (the one who calls us) is holy, we need to be holy in all manner of our conduct (KJV = conversation). Peter's rationale comes from the Old Testament. "Be ye holy; for I am holy."
If we have been called by God into a personal relationship with Him and we are trying to do those things that are pleasing to Him, we should desire to follow this command and seek to be holy. But what does it mean to be holy? There are definitions that we can apply to this situation. I personally like the idea that holiness is being set apart for God's service. Yet, that doesn't necessarily point us to the details of how we can be holy. Instead, I think the best way to find out what holiness entails is to go back to the sources of Peter's quote and look at the contexts of those verses.
The first appearance of "Be ye holy as I am holy" is found in Leviticus 11. This is the chapter that outlines what foods are acceptable and what are not.
Leviticus 11:43-45 (KJV)
Ye shall not make yourselves abominable with any creeping thing that creepeth, neither shall ye make yourselves unclean with them, that ye should be defiled thereby.
For I am the LORD your God: ye shall therefore sanctify yourselves, and ye shall be holy; for I am holy: neither shall ye defile yourselves with any manner of creeping thing that creepeth upon the earth.
For I am the LORD that bringeth you up out of the land of Egypt, to be your God: ye shall therefore be holy, for I am holy.
In verse 43, God told the Israelites not make themselves abominable by eating any of the creeping things that were described in verse 42, because then they would be defiled. Verse 44 relates the defiling of themselves and how instead they were to be sanctified and made holy, in the same way that God is holy. Verse 45 repeats "Be holy for I am holy" and states that this is required because God brought them out of Egypt and He is their God.
The second appearance of "Be ye holy as I am holy" comes from Leviticus 19.
Leviticus 19:1-4 (KJV)
And the LORD spake unto Moses, saying,
Speak unto all the congregation of the children of Israel, and say unto them, Ye shall be holy: for I the LORD your God am holy.
Ye shall fear every man his mother, and his father, and keep my sabbaths: I am the LORD your God.
Turn ye not unto idols, nor make to yourselves molten gods: I am the LORD your God.
In this passage God begins by stating the command to be holy and then begins a list of things that the Israelites were to do. Verse 3 speaks about honoring parents and keeping the sabbaths. Verse 4 is a prohibition of turning to and making idols. Again, the reason is that God is the God of the Israelites.
The last appearance of "Be ye holy as I am holy" comes from Leviticus 20.
Leviticus 20:7-8 (KJV)
Sanctify yourselves therefore, and be ye holy: for I am the LORD your God.
And ye shall keep my statutes, and do them: I am the LORD which sanctify you.
Surrounded by various commands, God inserts His requirement for sanctification and holiness. Verse 8 finishes the thought by saying that the way to fulfill this requirement is by keeping and doing God's statutes.
When Peter wrote his first epistle, he reiterated God's requirement to be holy. However, he didn't give a full explanation. He relied on a quote that he expected his readers to be familiar with. The way of holiness lay in obedience to the Word of God. But what was the Word of God at the time of Peter's writing? It was the Old Testament. It contained all that was needed to know regarding how to be holy.
Do we dare ignore the Old Testament's definition of holiness, in part, by refusing to acknowledge the Ten Commandments, by eating pork, and by ignoring the seventh day sabbath? We do so at our own risk. If God truly is our God we need to obey all His commands.