“Scripture speaks of both a holiness which we have in Christ before God, and a holiness which we are to strive after. These two aspects of holiness complement one another, for our salvation is a salvation to holiness: ‘For God did not call us to be impure but to live a holy life’ (1 Thessalonians 4:7). To the Corinthians, Paul wrote: ‘To the church of God in Corinth, to those sanctified in Christ Jesus and called to be holy’ (1 Corinthians 1:2). The word sanctified here means “made holy.” That is, we are through Christ made holy in our standing before God and called to be holy in our daily lives.
So the writer of Hebrews is telling us to take seriously the necessity of personal, practical holiness. When the Holy Spirit comes into our lives at our salvation, he comes to make us holy in practice. If there is not, then, at least a yearning in our hearts to live a holy life pleasing to God, we need to seriously question whether our faith in Christ is genuine.

The whole purpose of our salvation is that we be “holy and blameless in his sight” (Ephesians 1:4). To continue to live in sin as a Christian is to go contrary to God’s very purpose for our salvation… Holiness, then, is not necessary as a condition of salvation -that would be salvation by works – but as a part of salvation that is received by faith in Christ… We cannot receive half of God’s grace. If we have experienced it at all, we will experience not only forgiveness of our sins but also freedom from sin’s dominion.
This is the point James is making in his hard-to-understand passage on faith and works (James 2:14-26). He is simply telling us that a “faith” that does not result in works – in a holy life, in other words – is not a living faith by a dead one, no better than that which the demons possess.”
-Taken from Chapter 3, pp. 32-35