Definition and Meaning:
Sanctification is the process by which God sets apart individuals or groups for His holy purposes, transforming them into His likeness. It involves both a definitive act and a progressive work. The term "sanctification" comes from the Hebrew word "qadash" and the Greek word "hagiasmos," both of which mean to be set apart or made holy.

Biblical Foundation:
Sanctification is a central theme throughout Scripture, emphasizing God's desire for His people to be holy as He is holy. In the Old Testament, sanctification often involved ceremonial cleansing and dedication to God. In the New Testament, it takes on a more profound spiritual dimension, focusing on moral and spiritual purity.

Old Testament Context:
In the Old Testament, sanctification is closely associated with the covenant relationship between God and Israel. God declares His people to be holy, as seen in Leviticus 20:7-8 : "Consecrate yourselves, therefore, and be holy, because I am the LORD your God. Keep My statutes and practice them. I am the LORD who sanctifies you." Here, sanctification is both a divine act and a human responsibility.

The sanctification of objects, places, and people is evident in the consecration of the Tabernacle, the priesthood, and the nation of Israel itself. Exodus 29:44 states, "So I will consecrate the Tent of Meeting and the altar, and I will consecrate Aaron and his sons to serve Me as priests."

New Testament Context:
In the New Testament, sanctification is primarily the work of the Holy Spirit in the life of the believer. It is both positional and progressive. Positional sanctification occurs at the moment of salvation, where believers are set apart for God. Progressive sanctification is the ongoing process of becoming more like Christ.

1 Thessalonians 5:23 highlights this dual aspect: "Now may the God of peace Himself sanctify you completely, and may your entire spirit, soul, and body be kept blameless at the coming of our Lord Jesus Christ." This verse underscores God's active role in sanctification, ensuring the believer's complete transformation.

Role of Jesus Christ:
Jesus Christ is central to the believer's sanctification. Through His sacrificial death and resurrection, He provides the means for sanctification. Hebrews 10:10 states, "And by that will, we have been sanctified through the sacrifice of the body of Jesus Christ once for all." Christ's atoning work is the foundation upon which sanctification is built.

The Holy Spirit's Work:
The Holy Spirit plays a crucial role in the sanctification process. He indwells believers, empowering them to live holy lives. 2 Thessalonians 2:13 affirms this: "But we should always thank God for you, brothers who are loved by the Lord, because God has chosen you from the beginning to be saved by the sanctification of the Spirit and by faith in the truth."

Human Responsibility:
While sanctification is primarily a divine work, believers are called to participate actively. This involves obedience to God's Word, prayer, and the pursuit of holiness. Romans 12:1-2 exhorts believers: "Therefore I urge you, brothers, on account of God’s mercy, to offer your bodies as living sacrifices, holy and pleasing to God, which is your spiritual service of worship. Do not be conformed to this world, but be transformed by the renewing of your mind."

Eschatological Aspect:
Sanctification has an eschatological dimension, pointing to the future perfection of believers. Philippians 1:6 assures believers of God's faithfulness: "being confident of this, that He who began a good work in you will continue to perfect it until the day of Christ Jesus." This future aspect of sanctification culminates in glorification, where believers are fully conformed to the image of Christ.

Conclusion:
Sanctification by the LORD is a profound and multifaceted doctrine that encompasses God's sovereign work, the redemptive work of Christ, the empowering presence of the Holy Spirit, and the believer's active participation. It is a journey toward holiness, rooted in God's eternal purpose and culminating in the believer's ultimate glorification.