The call to pursue holiness is the call to obedience as we are sanctified by the power of the Holy Spirit. The English word sanctification as it is used in the New Testament is a translation of the word ἁγιασμός which comes from the root word άγιος denoting holiness. In other words, to be sanctified is to be made holy! Therefore, understanding the process of sanctification begins with understanding holiness. Defining holiness can be difficult as it is a term that may be defined in many ways. In fact, it is a term that is defined vaguely by the Westminster Dictionary as a “peculiar quality of the religious.” For the Christian attempting to define holiness from scripture, I would define it as all of that which relates to who God is. This is why Peter says we are called to be Holy as God is Holy in his quotation from Leviticus. (1 Peter 1:16) The aim of growing in holiness is to be a progressively clearer reflection of who God is.
In Colossians 3, Paul is contrasting what he refers to as the “earthly nature” i.e., old self, with the new self. Paul says that this new self is being renewed into the image of its creator. The language he uses indicates that in our sanctification we are being renewed into the image of Christ. Here, we have an excellent definition of holiness: “renewed in knowledge after the image of its creator.” (Colossians 3:10) To be made holy is the process of being transformed into the image of the triune God. There is no true definition of holiness outside of God — it is only in God himself that the term finds meaning.
Jesus Christ is the full revelation of God in the flesh; thus, the way in which we are made holy is following him in obedience. In following Christ, we are growing in holiness as we are conformed by the work of the Spirit into the image of Christ. This means first and foremost that we increasingly share in the character of Christ—holiness is to have Christlike character. Being like Christ means that we share in his attitudes and actions. Take a moment to think through some of the attitudes and actions mentioned by Paul in his imperative given to the church in Colossae. They were called to put to death: sexual immorality, impurity, lust, evil desires, greed, anger, rage, malice, filthy language. (Colossians 3:5-8) These things are incompatible with the character of Christ and therefore incompatible with anyone claiming to be a follower of Christ. Holiness is not only the absence of such things — for it not only puts to death that which is contrary to the character of Christ, but it also puts on that which is consistent with the character of Christ — compassion, kindness, humility, meekness, patience, forgiveness, forbearance, love, & gratitude. (Colossians 3:12-16)