In 1 Thessalonians 5:23-24 Paul says,
“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.”
I like verse 24 that says, “The one who calls you is faithful, and he will do it.” God didn’t call us to something that can’t be accomplished. To be sanctified means to be made holy and set apart for a purpose.
The Amplified version reads like this,
“Now may the God of peace Himself sanctify you through and through [that is, separate you from profane and vulgar things, make you pure and whole and undamaged—consecrated to Him—set apart for His purpose]; and may your spirit and soul and body be kept complete and [be found] blameless at the coming of our Lord Jesus Christ. Faithful and absolutely trustworthy is He who is calling you [to Himself for your salvation], and He will do it [He will fulfill His call by making you holy, guarding you, watching over you, and protecting you as His own].”
“That is, separate you from profane and vulgar things, make you pure and whole and undamaged,” There are plenty of people who think they can never be pure, whole and undamaged, but that’s just because they have the wrong information.
Hebrews 10:10 says,
“By that will we have been sanctified through the offering of the body of Jesus Christ once for all.”
Paul says in 1 Corinthians 6:11,
“But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.”
The Amplified version reads like this:
“But you were washed [by the atoning sacrifice of Christ], you were sanctified [set apart for God, and made holy], you were justified [declared free of guilt] in the name of the Lord Jesus Christ and in the [Holy] Spirit of our God [the source of the believer’s new life and changed behavior].”
God doesn’t call us to something without giving us the capacity for it. To be sanctified is to be made holy, and to be set apart for the purposes of God. That’s what we ARE, we who believe.