Sanctification is not a negative matter: “Don’t do this” and “Don’t do that.” It is rather the positive truth that God wants us for Himself as a sacred possession, much as a bridegroom considers his bride his very own in a special, sacred way.
Bible sanctification is a twofold truth, affecting both our standing before God and our spiritual state. In one sense every true believer in Christ has already been sanctified, or consecrated to God, by the operation of the Holy Spirit. Thus we read:
“…God hath from the beginning chosen you to salvation through sanctification of the Spirit…” (II Thes. 2: 13).
“Elect according to the foreknowledge of God the Father, through sanctification of the Spirit…” (IPet.1:2).
This has nothing to do with our conduct. God did it. Sanctification begins with Him. Thus Paul could write to even the careless Corinthian believers and say: “Ye are sanctified” (1 Cor. 6:11; cf. Acts 20:32; 26:18), i.e., “God has set you apart for Himself.” This phase of sanctification is based on the redemptive work of Christ in our behalf, for Heb. 10:10 says: “We are sanctified through the offering of the body of Jesus Christ once for all.”
But now God would have us appreciate this fact and conduct ourselves accordingly, consecrating ourselves ever more completely to Him. This is practical, progressive sanctification. “For this is the will of God, even your sanctification” (I Thes. 4:3). Hence Paul’s benediction: “The very God of peace sanctify you wholly” (I Thes. 5:23), and his exhortation to Timothy to be “a vessel unto honor, sanctified, and meet [fit] for the Master’s use” (II Tim. 2:21).
How can believers be more wholly sanctified to God in their practical experience? By studying and meditating on His Word. Our Lord prayed: “Sanctify them through Thy truth: Thy Word is truth” (John 17:17), and Paul declares that “Christ… loved the Church and gave Himself for it, that He might sanctify and cleanse it with the washing of water by the Word” (Eph. 5:25,26). source