Apostle Peter said in 1st Peter 1:15-16, "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." I would guess that Apostle Peter was referring to Leviticus 11:44-45 or Leviticus 19:2 or perhaps both. The word "holy," in the Hebrew is "קדשׁ קדושׁ qâdôsh qâdôsh kaw-doshe', kaw-doshe' from H6942; sacred (ceremonially or morally); (as noun) God (by eminence), an angel, a saint, a sanctuary: - holy (One), saint." Likewise in the Greek the word "holy," is "ἅγιος hagios hag'-ee-os From ἅγος hagos (an awful thing) compare G53, [H2282]; sacred (physically pure, morally blameless or religious, ceremonially consecrated): - (most) holy (one, thing), saint." from G40. The Bible is clear, God wants Christians to be a holy people.
But what is holiness? If holiness is a style of dress, perhaps our UPCI missionaries should Americanize all the foreign converts to look like Westerners. But would that make them holy? There are too many American Christians who don't meet our perception of holiness much less the Bible definition. There are those who might say that holiness is only of the heart. They seem to invariably use the passage found in 1st Samuel 16:7 where the Bible says, "the LORD looketh on the heart." This, of course, was when Samuel went to the home of Jesse to anoint the king that God had chosen. What the entire verse says is, "But the LORD said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart." Neither Samuel or God was dealing with dress style but with whom God had chosen to be king of Israel. We are, however to "Abstain from all appearance of evil." (See 1st Thessalonians 5:22.")
So what is holiness? I have heard over the years preachers referencing the scripture found in Hebrews 12:14 saying, "without holiness no man shall see the Lord." Fortunately, and perhaps unfortunately for them, what the Bible does say here is "Follow peace with all men, and holiness, without which no man shall see the Lord..." "Follow peace with all men, and holiness?" Many people, both saints and preachers, live what they think is a holy lifestyle when in fact they are merely dressing a certain way all the while they will hardly look at those who may disagree in some point or other. My prayer is that God will have mercy on their souls.
What is holiness? We used to sing that beautiful old chorus, "To Be Like Jesus," the words of which go like this;
To be like Jesus, to be like Jesus
On earth I long to be like Him
All through life's journey, from earth to glory
I only ask to be like Him.
Truly, to be like Jesus would fit the scriptures found in Leviticus and 1st Peter. But how did the Lord Jesus dress? Did he have a suit and tie? If we really wanted to be exactly like the Lord, we would be totally submitted to our Father and at the end of our lives go to the cross. The Lord Jesus said in Luke 9:23 "And he said to them all, If any man will come after me, let him deny himself, and take up his cross daily, and follow me." Walking with Jesus is to take up our cross daily and follow Him. Most of us would rather serve God in some other easier way.
Holiness is a lifestyle! To be holy as God is holy, one must get up in the morning with the sole desire to live for and to please God. There is an old saying I remember from a child. "Politeness is to do and say the kindest things in the kindest way," (something like that anyway). Holiness is, throughout each day, to do and say the words and deeds that have been submitted to God for approval. I am reminded of a verse of scripture found in Colossians 3:17, "And whatsoever ye do in word or deed, do all in the name of the Lord Jesus, giving thanks to God and the Father by him." The Lord Jesus said in John 13:35, "By this shall all men know that ye are my disciples, if ye have love one to another." To love the brethren is to be holy. (1st John 3:14 "...He that loveth not his brother abideth in death")