Sanctification is one of those religious sounding words that people in the church use, but may not understand what it means. In its most basic sense, to sanctify something is to set it apart for God’s special use and purpose. Therefore, God’s people are sometimes said to be sanctified because they are set apart for God’s special purposes in the world: “Consecrate yourselves, therefore, and be holy; for I am the LORD your God. Keep my statutes, and do them; I am the LORD who sanctifies you” (Lev 20:7–8). As this verse helps us see, the word sanctification is closely related to holiness. The word sanctification can be used in a similar sense in the New Testament, and in one sense, believers are already sanctified because of what Jesus has done for them. Hebrews 10:10 tells us that as followers of Christ “we have been sanctified through the offering of the body of Jesus Christ once for all.”
But most of the time when Christians use the word sanctification, they are referring to the progressive work of God to make a believer more like Jesus Christ. As such Paul can write, “For this is the will of God, your sanctification: that you abstain from sexual immorality” (1 Thes 4:3). He commands Christians to “present your members as slaves to righteousness leading to sanctification” (Rom 6:19). Because believers have been set free from their slavery to sin, “the fruit you get leads to sanctification and its end, eternal life” (Rom 6:22).
The Greek word translated “sanctification” (hagiasmos [aJgiasmov”]) means “holiness.” To sanctify, therefore, means “to make holy.” In one sense only God is holy ( Isa 6:3 ). God is separate, distinct, other. No human being or thing shares the holiness of God’s essential nature. There is one God. Yet Scripture speaks about holy things. Moreover, God calls human beings to be holy — as holy as he is holy ( Lev 11:44 ; Matt 5:48 ; 1 Peter 1:15-16 ). Another word for a holy person is “saint” (hagios [a&gio”]), meaning a sanctified one. The opposite of sanctified is “profane” ( Lev 10:10 ).
From time to time human beings are commanded to sanctify themselves. For example, God commanded the nation of Israel, “consecrate to me every firstborn male” ( Exod 13:2 ). God said through Peter, “in your hearts set apart Christ as Lord” ( 1 Peter 3:15 ). One sanctifies Christ by responding to unbelievers meaningfully, out of a good conscience and faithful life. God calls his own to set themselves apart for that which he has set them apart. Sanctify, therefore, becomes a synonym for “trust and obey” ( Isa 29:23 ). Another name for this action is “consecration.” To fail to sanctify God has serious consequences ( Num 20:12 ).
Human beings ultimately cannot sanctify themselves. The Triune God sanctifies. The Father sanctifies ( 1 Cor 1:30 ) by the Spirit ( 2 Thess 2:13 ; 1 Peter 1:2 ) and in the name of Christ ( 1 Cor 6:11 ). Yet Christian faith is not merely passive. Paul calls for active trust and obedience when he says, “Since we have these promises, dear friends, let us purify ourselves from everything that contaminates body and spirit, perfecting holiness out of reverence for God” ( 2 Cor 7:1 ). No one may presume on God’s grace in sanctification. Peter reminds believers to be diligent in making their calling and election sure ( 2 Peter 1:10 ).
A person or thing can be sanctified in two ways according to God’s creative purpose or according to God’s redemptive design. All sanctified in the first sense are used by God in the second sense. Not all God uses in the second sense are sanctified in the first sense.
What is Sanctification by Nathan Bingham https://www.ligonier.org/blog/what-sanctification/
Sanctification is a Direction by David Powlison https://www.crossway.org/articles/sanctification-is-a-direction/
6 things every Christian needs to know about sanctification by Coleman Ford https://equip.sbts.edu/article/6-things-every-christian-needs-know-sanctification/