Easton’s Bible Dictionary
Sanctification:
involves more than a mere moral reformation of character, brought about by the power of the truth: it is the work of the Holy Spirit bringing the whole nature more and more under the influences of the new gracious principles implanted in the soul in regeneration. End quote
Given that sanctification does not occur until regeneration then what it means to the Old Testament economy is different since back then the Holy Spirit was an external element of relationship with God the Father.
“You are to speak to the people of Israel and say, ‘Above all you shall keep my Sabbaths, for this is a sign between me and you throughout your generations, that you may know that I, the Lord, sanctify you.
Keeping the Sabbath was a sign that they were sanctified and dependent on their faithfulness to obey. When they stopped keeping the Sabbath it was a sign that God no longer held them to be holy in His sight.
And Jehu ordered, “Sanctify a solemn assembly for Baal.” So they proclaimed it.
They took a godly endeavor and applied it to false worship. Perhaps that is why the word sanctify only appeared three times in the Old Testament before the words of the prophet Isaiah.
“Those who sanctify and purify themselves to go into the gardens, following one in the midst, eating pig’s flesh and the abomination and mice, shall come to an end together, declares the Lord.
Defile a holy thing and die was the warning. They failed over and over again until Christ came and put an end to self-effort as a way to obtain a right standing before God.
- John 17:17
Sanctify them in the truth; your word is truth. - Ephesians 5:26
that he might sanctify her, having cleansed her by the washing of water with the word, - 1 Thessalonians 5:23
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.