The concept of sanctification is found throughout Scripture and in reference to a variety of subjects. For example, God sanctifies the seventh day in Genesis 2, Moses and the people sanctify the priests in Exodus 28, Jesus commands us to pray that God would sanctify his name in Matthew 6, and so on. But though the subjects and circumstances can vary, the general meaning of the word remains consistent. To sanctify (the process of which we call sanctification) means to render someone or something unique, to set it apart or make it holy.
In Christian theology, the term sanctification is used most often to describe the setting apart or making holy of Christians. After being justified and adopted by God, Christians begin a process in which, through the power of the Holy Spirit, they are incrementally transformed in every aspect into the likeness of Jesus Christ.
One of the clearest passages on this is Paul’s prayer at the end of 1 Thessalonians:
Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ. He who calls you is faithful; he will surely do it. (1 Thessalonians 5:23-24)
Among the many wonderful things that can (and should) be said about sanctification, perhaps the most important and most encouraging is what Paul makes clear here: sanctification is an act of God, and having begun the work, he will always be faithful to bring it to completion.