Introduction
Sanctification is God’s will for us (1 Thessalonians 4:3). The word sanctification is related to the word saint; both words have to do with holiness. To “sanctify” something is to set it apart for special use; to “sanctify” a person is to make them holy.
Jesus had a lot to say about sanctification in John 17. In verse 16 the Lord says, “They are not of the world, even as I am not of it,” and this is before His request: “Sanctify them by the truth; your word is truth” (verse 17). In Christian theology, sanctification is a state of separation unto God; all believers enter into this state when they are born of God: “You are in Christ Jesus, who became to us wisdom from God, righteousness and sanctification and redemption” (1 Corinthians 1:30, ESV). The sanctification mentioned in this verse is a once-for-ever separation of believers unto God. It is a work God performs, an integral part of our salvation and our connection with Christ (Hebrews 10:10). Theologians sometimes refer to this state of holiness before God as “positional” sanctification; it is related to justification.
From: https://www.gotquestions.org/sanctification.html