The definition of Sanctification:
The state of growing in divine grace because of Christian commitment after baptism or conversion (Merriam-Webster)
In the first instance, to sanctify is to regard or declare “Holy” by separating from common usage to the service of God. (Funk & Wagnall’s Bible dictionary)
Hebrews 13:12 (NKJV)
Therefore, Jesus also, that He might sanctify the people with His own blood, suffered outside the gate.
Ephesians 5:26-27 (NRSV)
to make her holy by cleansing her with the washing of water by the word, so as to present the church to himself in splendor, without a spot or wrinkle or anything of the kind, yes, so that she may be holy and without blemish.
Ephesians 4:23-24 (NIV2011)
to be made new in the attitude of your minds; and to put on the new self, created to be like God in true righteousness and holiness.
Sanctification is not some magic trick that God uses to keep us in line. It is the act of regeneration that leads to sanctification. Regeneration is the act of studying God’s word to the point of building a good relationship with God through the Holy Spirit.
We are not sanctified by the act of baptism, laying on of hands, or with some class that we attend. Only the Holy Spirit can help us achieve sanctification. Notice I did not say the Holy Spirit would give us sanctification. The Holy Spirit will only lead us and help us to achieve sanctification.
Why do we need sanctification? Without sanctification, we are just another person in the large pool of society. Just another person bound for the inevitable pool of sinners bound for the permanent punishment of the wicked.
To have sanctification is to be part of a small group of Christians that will live with God in Heaven.