What does the Church of God believe regarding sanctification?
Key Verse:  Sanctify them through thy truth: thy word is truth.  [John 17:17]

Sanctification is commanded for the people of God.  To be sanctified means to be set apart for a holy purpose.

To open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me.  [Acts 26:18]

Jesus has a deep concern and love toward man, and it is that we would be set apart for his holy purpose, without Jesus we are living in darkness and sin.

And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.  [I Corinthians 6:11]

The Lord wants us to inherit his kingdom but we must be set apart (sanctified) and not be as we were before we came into the light of His truth.

If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master's use, and prepared unto every good work.  [2 Timothy 2:21]

The Lord calls us to be a vessel unto honour filled with His Holy power and prepared unto every good work, when this takes place we become sanctified and set apart for a holy purpose.

Wherefore Jesus also, that he might sanctify the people with his own blood, suffered without the gate.  [Hebrews 13:12]

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:  [1 Peter 3:15]

We should always be able to give an answer to man of the hope that we have through the truth which is the love of God, we are told to love our neighbor and giving them the truth is God's love.

Jude, the servant of Jesus Christ, and brother of James, to them that are sanctified by God the Father, and preserved in Jesus Christ, and called:  [Jude 1:1]

For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren,  [Hebrews 2:11]

By the which will we are sanctified through the offering of the body of Jesus Christ once for all.  [Hebrews 10:10]

Jesus gave His life for you and I so that we may be able to be sanctified and carry out the mission that He left for us, which is the truth and example of living as true children of God.