To sanctify is to be set apart. The very first time the God gave His commandments, He required that the firstborn of all man (and beast) be set apart (sanctified) for him because He claimed them to be His(Exodus 13:2) and then He told Moses to go and sanctify His people and the priests by washing their clothes before facing God (Exodus 19:10, 22) but Moses responded by saying, that the people could not come to face God even if cleansed by their own accord (Exodus 19:23). From the book of Exodus throughout the old testament, there are many times, the voice of God is heard either directly from Him or through His prophets, requiring that one sanctify himself/herself, but when we come to the New Testament, all the instances of the need to be sanctified is recorded as one that is not required of man, but as one that has been granted by God, because of His Son, Jesus Christ. See, we cannot sanctify ourselves by washing ourselves, but the blood of Jesus Christ that washes our inward evil self is the only thing that can sanctify us (Hebrews 13:12).
In the Lord’s prayer as recorded in John 17, Jesus prayed that God sanctifies us (John 17:17) and then He makes a bewildering statement. He said, I sanctify myself for the sake of my people (John 17:23). In other words, Jesus was saying that He was setting himself apart from God and His divinity for you and me, so that we can be set apart from our humanity and be more Christ like in spirit, soul and body. Our beings are sinful and our flesh that war against the Holy Spirit of God is corrupt and cannot be set apart on its own will, but it is the blood of Jesus alone that can set us apart. My prayer is that of Apostle Paul; that the very God of peace sanctify me wholly; and I pray that my whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ. (1 Thessalonians 5:23). Let this be yours as well!
Exodus 13:2 (KJV)
2 Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine.
Exodus 19:10-11,22-23 (KJV)
10 And the LORD said unto Moses, Go unto the people, and sanctify them to day and to morrow, and let them wash their clothes,
11 And be ready against the third day: for the third day the LORD will come down in the sight of all the people upon mount Sinai.
22 And let the priests also, which come near to the LORD, sanctify themselves, lest the LORD break forth upon them.
23 And Moses said unto the LORD, The people cannot come up to mount Sinai: for thou chargedst us, saying, Set bounds about the mount, and sanctify it.
John 17:17-19 (KJV)
17 Sanctify them through thy truth: thy word is truth.
19 And for their sakes I sanctify myself, that they also might be sanctified through the truth.
1 Thessalonians 5:23-24 (KJV)
23 And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.
24 Faithful is he that calleth you, who also will do it.