What Does Salvation in Sanctification of the Spirit Mean in 2 Thessalonians 2:13?

     Do you ever gloss over the word sanctification when you read it?  Paul talks about sanctification quite a bit in 1 & 2 Thessalonians. This week the phrase "salvation in sanctification of the Spirit" caught my attention: 

But we ought to thank God always concerning you, brothers beloved of the Lord, because God chose you from the beginning unto salvation in sanctification of the Spirit and belief of the truth.  

2 Thessalonians 2:13

And in chapter 4 in the Life-Study of 2 Thessalonians I liked Witness Lee's clear explanation of what sanctification in salvation actually means:   

God has selected us unto salvation in sanctification and not merely to have our sins forgiven….A person who prays in this way can be saved and have the forgiveness of sins. But this is not sanctification.  After we experience forgiveness, we need to undergo the process of sanctification. The Lord intends to sanctify us, to transform us….In verse 13 Paul tells us that sanctification is of the Spirit….You need to realize that the Spirit is in you to sanctify you…..The Spirit indwells you with just one goal---to sanctify us, to transform us, to change us metabolically. This is sanctification of the Spirit (Lee, Witness. Life-Study of 2 Thessalonians. Anaheim: Living Stream Ministry, 1982,  Print).

What spoke to me was that sanctification is a process that begins when we're regenerated. And every day the Lord is working to sanctify us, to spread His holy life and nature into all the parts of our being.     The line I bolded above really jumped out at me. I don't think I ever thought before that the Spirit's one goal was to totally transform me by sanctifying me day by day. 
    
    And I was just thinking to myself how does the Spirit sanctify me? And right away I remembered that awesome verse in John 17:17:  Sanctify them in the truth; Your word is truth! So now whenever I think about sanctification I think of God's Spirit saturating me through His word.

You can  also listen to live excerpts from these messages on 1 Thessalonians on Life-Study of the Bible with Witness Lee.  

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What Does Sanctification in Salvation Mean in 2 Thessalonians 2:13?