"Sanctification" is a fancy word for "becoming more like Jesus."
It's a process we begin when we make Christ Lord and it continues until we see Him face to face (1 Corinthians 13:12).
It's God's will for us:
"It is God's will that you should be sanctified" 1 Thessalonians 4:3
It's a dual process:
We have a role in sanctification and God has a role: "continue to work out your salvation with fear and trembling, for it is God who works in you to will and to act in order to fulfill his good purpose." Philippians 2:12-13
Working out our salvation means learning to yield our will to God's will--learning self-denial (Luke 9:23-24). "With fear and trembling" means doing it deliberately and diligently.
Romans 8 puts it this way: "we have an obligation" to live according to the Spirit and deny our flesh.
Our role is critical, but God does the hard part...He changes us inside out!