The word "sanctify" means to "set apart." It is to take something out of a group and set it apart. Biblically the word sanctify is used when something is set apart BY AND FOR GOD. God does not take it out of the group and set it in a corner. He takes it out of the group and sets it next to Him. When something is sanctified by God it is set apart from the world and put next to God - for God.
The first use of the term sanctified is here in Genesis. God set apart the seventh day as a day for Him. Later, when Moses was given the law, the sabbath was set apart by God as a day in which the children of Israel were to think about Him as their provider. They were not to work that day or think about the things of the world. They were to be thankful to God. It was a day set apart from the other days of the week BY AND FOR GOD. Sanctification is something God does.
Man always seems to corrupt the things of God. The children of Israel took the sanctification of one day of the week (as do many today) and made it a religious practice filled with ceremony and man's ways of worship that have little to do with being set apart by and for God. It became so bad that God said in Isaiah 1:13 "Bring no more vain oblations; incense is an abomination unto me [it is disgusting]; the new moons and sabbaths, the calling of assemblies, I cannot away with [I can't stand it!]; it is iniquity, even the solemn meeting."
God's Word is God's Will. God sets believers apart from the world by His Word.
I [Jesus Christ] have given them thy word; and the world hath hated them, because they are not of the world, even as I am not of the world.
I pray not that thou shouldest take them out of the world, but that thou shouldest keep them from the evil [one].
They are not of the world, even as I am not of the world.
Sanctify them through thy truth: thy word is truth.
We live in a corrupt world where the evil one [the Devil] holds power and authority. Yet by believing God's Word we are set apart from the world BY AND FOR GOD. The Devil has no rights over the person who believes God's Word and is thereby sanctified from the world and put next to God.
By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
Jesus Christ believed God and was beaten, crucified, and died as the only true offering to God for us. When we believe that Jesus Christ is Lord: That he went through this in our place - for us - and that God raised him from the dead we are sanctified by the will of God.
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.
Salvation is wholeness. Before we believe Romans 10:9-10 we are body and soul men and women and part of the world. We lack holy spirit which God intended from the beginning to be the third part of a whole man or woman. When we believe the Word of God as declared here in Romans, we receive the gift of holy spirit. We are saved - made whole. And we are set apart BY AND FOR GOD by the spirit we receive.
1 Peter 1:2
Elect according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ: Grace unto you, and peace, be multiplied.
Obedience to the Word of God, belief in the sacrifice of Jesus Christ for us and God's act of raising him from the dead results in our being set apart by God. God knows in His foreknowledge who is going to be obedient to His Word and He has chosen [elected] us to be set apart by giving us holy spirit when we believe.
2 Thessalonians 2:13
But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
We are set apart BY AND FOR GOD by receiving holy spirit when we believe the truth of God's Word.
1 Corinthians 1:30
But of him are ye in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption:
Sanctification is one of our legal rights as born again children of God. With wholeness comes wisdom from God; we are made right before God; we are bought back [redeemed] from the world; and we are set apart from the world BY AND FOR GOD.
1 Thessalonians 5:23
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.
Our sanctification is complete sanctification. We are sanctified by God wholly! Spirit, soul, and body - all three parts are set apart BY AND FOR GOD. The Devil has no legal rights over any part of the believer. Your physical body is set apart by and for God. The Devil has no right to make you sick. Your mind and soul is set apart for God. The Devil has no right to control your thoughts or any part of your life. Your spirit is set apart for God. You are now a spiritual being with direct access to God. We are wholly set apart BY AND FOR GOD.
So what is our part? We are to live a life that demonstrates our sanctification. That does not mean that we separate ourselves from others in the world who need what we have! It means that we are to set God apart for us in our heart. And when others ask us - "What makes you so positive? What is the focus and goal that you are looking toward that keeps you so separate from the trouble of this world?" We are to be ready to give them the answer. We set God apart in our heart and look toward that glorious day when Christ shall return. We live a life set apart from the world knowing that one day we will be taken out of this world just as we are already set apart BY AND FOR GOD.
1 Peter 3:15
But sanctify the Lord God in your hearts: and be ready always to give an answer to every man that asketh you a reason of the hope that is in you with meekness and fear: