Set apart for God
To sanctify means to set apart for a holy purpose. Positional sanctification describes a believer's standing before God: they now belong to him, are counted among his holy people, and have been brought into a new relationship through Jesus.
Paul addressed the troubled church in Corinth as people sanctified in Christ Jesus and called to be saints, even though the letter goes on to correct serious sins and divisions among them (1 Corinthians 1:2). Their behaviour needed attention, but their basic identity in Christ had already been given.
Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours:
A gift secured by Christ
This standing rests on what Jesus has done, not on a person's ability to become faultless. Hebrews connects sanctification with Christ offering himself once for his people (Hebrews 10:10). The same chapter says that his single offering has accomplished what repeated sacrifices could not do for those being made holy (Hebrews 10:14).
Paul puts sanctification alongside being washed and justified when he reminds the Corinthians of the change God had brought about in them (1 Corinthians 6:11). Positional sanctification is therefore closely tied to salvation. God receives believers as his own because of Christ.
By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
Not the same as daily growth
Christians often use the same word, sanctification, for another reality too: the lifelong process of becoming more like Christ in thought, desire, and action. That process involves real choices, repentance, prayer, Scripture, and the help of God's Spirit. It is uneven at times. A believer may know they belong to God while still facing stubborn habits and wounds.
That is why Paul can speak of believers as already sanctified while also urging them to offer themselves to God for righteous living (Romans 6:19). Positional sanctification answers, “Whose am I?” Progressive sanctification asks, “How will I live as someone who belongs to God?” Both matter, but they are not the same thing.
Why this brings hope
This teaching does not excuse sin. Instead, it gives a secure place from which to fight it. You do not pursue holiness in order to persuade God to accept you. In Christ, you pursue it because God has set you apart for himself.
When you feel discouraged by slow growth, remember that the New Testament speaks to ordinary, imperfect believers as God's holy people. Keep bringing your life honestly to God, and keep reading the passages around these verses. The God who sets his people apart also calls them to walk in a way that reflects their new identity (1 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.