The one who calls you is faithful and he will do it
1 Thessalonians 5:23-24 (NIV) May God himself, the God of peace, sanctify you through and through. May your whole spirit, soul and body be kept blameless at the coming of our Lord Jesus Christ. The one who calls you is faithful and he will do it.
Paul concluded his letter to the Thessalonians with a summary of all that he had been expressing in detail. His desire is that God sanctify them completely. To sanctify is to set apart for the sole use of God. It means to be holy. The verb tense is aorist, active, optative. Expanding that meaning, we might translate in the following way. It is my desire for something that may never come to completion, that is that God, having begun to purify you for Himself, might continue to do so.
The optative mood had almost disappeared from usage at the time this was written. It indicates a desire for an action to occur in which the fulfillment is doubtful. Yet, Paul added that the One who called them is faithful and will do it. Paul mentioned in the letter to the Philippians that he had not yet attained perfection. But this is what God is faithfully doing in our lives. It is God who is at work sanctifying you. Though Paul has just listed a number of commands that are helpful in the process, it is a work of God.
This passage mentions the three parts of man. Paul is praying that each be sanctified completely. The spirit is the place in us that is born to new life and communion with God. Without the new birth, this part of us is void and empty. The soul is our mind, will, and emotions. If we let the Spirit direct our lives from within our own spirit, our soul will submit to and serve Him. But when the soul is leading, we will walk in our carnal nature. The sanctification of the soul means the complete identification with the Spirit's thoughts, desires, and emotions. As that is increasingly complete, the body then acts in accordance with the sanctified soul. Our actions are the final expression of the completeness of the sanctification process. That is why Jesus declared that you can know a tree by its fruit.
Consider: Don't get discouraged if you are just beginning the process, God is the One at work faithfully completing what He started.