The definition of holiness includes the concept of being distinct, set apart from everything (or everyone) else. Christians are holy people in two ways. First, believers are holy in God’s sight in terms of their position. Through faith in Christ, they are uniquely set apart by God, and for God. In this sense, no true Christian can be any more holy than he already is.
The Bible also speaks of the need for consistent and increasing holiness in behavior. Therefore, the holy person practices righteousness rather than sin, lives in purity rather than uncleanness, is godly rather than worldly.
Consider four facts from the Bible about holiness in behavior:
Holiness is not optional
“Pursue peace with all men, and the sanctification [or “holiness”] without which no one will see the Lord” (Heb. 12:14). The two commands in this verse may initially appear to conflict with each other: get along with all people and be holy in your behavior. If we allow peace with the people of this world to be our overriding concern, we might easily be lax in living according to God’s standard of holiness. The practice of holiness, after all, has the tendency to offend. So, as it seems, the writer saw the need to qualify his first command with a second: be holy in your behavior even if it causes conflict with others.
The reason for giving the second command priority over the first is seen in the last phrase: “without which [i.e., without holiness] no one will see the Lord.” You may complete your life, as many Christian martyrs did, as the apostles did, and as Jesus Himself did, amid great animosity from most people around you, and still be confident of your entrance into heaven. Yet, if you die as one whose behavior was not consistently and increasingly holy—not set apart from the worldliness all around you—then as this text makes plain, you will not “see the Lord.” Instead you will hear, “I never knew you. Depart from Me, you who practice lawlessness” (Matt. 7:23).
Holiness is required in every corner of your life
As obedient children, do not be conformed to the former lusts which were yours in your ignorance, but like the Holy One who called you, be holy yourselves also in all your behavior; because it is written, “You shall be holy, for I am holy.” (1 Pet. 1:14-16)
Note the words, “in all your behavior.” In this text, Christian freedom has its boundary. You are free to live as you want to live, and do as you want to do—as long as all of your behavior is holy. And by holy, Peter clearly directs us to imitate God. Would God approve of the things you allow yourself to do, say, think, listen to, or watch? In your ignorance as an unbeliever you felt no qualms about engaging in less-than-holy behavior. But what about now, as one who has been set apart for God? Are you pursuing holiness in every area, or are there still dark corners of your life where you are taking liberties you shouldn’t?
Holiness has only one instruction manual: the Bible
The psalmist asks a simple yet profound question: “How can a young man keep his way pure?” (Ps. 119:9). This was no reflection of idle curiosity, but rather an expression of urgent need and conviction. He longed to be pure (holy, righteous) in the way he conducted his life, and so he asked how this can be done. He then gave the only acceptable answer: “By keeping it according to Your word.” Holiness is not the product of intuition, but of instruction. The person who walks in holiness is the one who knows and obeys his Bible.
The practice of holiness is one and the same as a vigorous spiritual life. A person cannot have the one without the other, and the person who has the one also has the other. Both flow from one’s relationship with Scripture. As the 19th century man of faith, George Muller, once said, “the vigor of our spiritual life will be in exact proportion to the place held by the Bible in our life and thoughts.”
Holiness requires discretion
Paul instructs us to take pains to avoid even being tempted to sin. “Make no provision for the flesh in regard to its lusts” (Rom. 13:14). This is a command to exercise discretion—to recognize the seductive, deceptive nature of sin and the unholy desires of your flesh that still tend to draw you toward sin, and then to make every effort to keep the two apart. If you want to avoid a deadly explosion, you will keep open flames away from gasoline vapor. Likewise, if you want to avoid sinning, you will be careful to avoid unnecessarily placing yourself in the presence of temptation. If our only command were to avoid committing the sin itself, there would be no violation in simply drawing near, as long as we did not actually succumb. But with Paul’s command in Romans 13:14, we sin when we get careless.
The best course to prevent falling into the pit is to keep at the greatest distance. He that will be so bold as to attempt to dance on the brink of the pit, may find by woeful experience that it is a righteous thing with God that he should fall into the pit.”
 Thomas Brooks, Precious Remedies Against Satan’s Devices, 1652.