Question: "What is sanctification? What is the definition of Christian sanctification?"
Answer: Jesus had a lot to say about sanctification in the Book of John, chapter 17. In verse 16 the Lord says, “They are not of the world, even as I am not of the world,” and this is before His request: “Sanctify them in the truth: Thy word is truth.” Sanctification is a state of separation unto God; all believers enter into this state when they are born of God: “But of Him you are in Christ Jesus, who became for us wisdom from God—and righteousness and sanctification and redemption” (1 Corinthians 1:30). This is a once-for-ever separation, eternally unto God. It is an intricate part of our salvation, our connection with Christ (Hebrews 10:10).
Sanctification also refers to the practical experience of this separation unto God, being the effect of obedience to the Word of God in one’s life, and is to be pursued by the believer earnestly (1 Peter 1:15; Hebrews 12:14). Just as the Lord prayed in John 17, it has in view the setting apart of believers for the purpose for which they are sent into the world: “As Thou didst send Me into the world, even so send I them into the world. And for their sakes I sanctify Myself, that they themselves also may be sanctified in truth” (v. 18, 19). That He set Himself apart for the purpose for which He was sent is both the basis and the condition of our being set apart for that for which we are sent (John 10:36). His sanctification is the pattern of, and the power for, ours. The sending and the sanctifying are inseparable. On this account they are called saints, hagioi in the Greek; “sanctified ones.” Whereas previously their behavior bore witness to their standing in the world in separation from God, now their behavior should bear witness to their standing before God in separation from the world.
There is one more sense that the word sanctification is referred to in Scripture. Paul prayed in 1 Thessalonians 5:23, “The God of peace Himself sanctify you wholly; and may your spirit and soul and body be preserved entire, without blame at the coming of our Lord Jesus Christ.” Paul also wrote in Colossians of “the hope which is laid up for you in the heavens, whereof ye heard before in the word of the truth of the Gospel” (Colossians 1:5). He later speaks of Christ Himself as “the hope of glory” (Colossians 1:27) and then mentions the fact of that hope when he says, “When Christ, who is our Life, shall be manifested, then shall ye also with Him be manifested in glory” (Colossians 3:4). This glorified state will be our ultimate separation from sin, total sanctification in every aspect. “Beloved, now we are children of God; and it has not yet been revealed what we shall be, but we know that when He is revealed, we shall be like Him, for we shall see Him as He is” (1 John 3:2).
To summarize, sanctification is the same Greek word as holiness, “hagios,” meaning a separation. First, a once-for-all positional separation unto Christ at our salvation. Second, a practical progressive holiness in a believer’s life while awaiting the return of Christ. Third, we will be changed into His perfect likeness—holy, sanctified, and completely separated from the presence of evil.