Answer
Sanctification is both the work of the Holy Spirit and believers. So, on the one hand, we want to affirm that it's only God who makes us holy. God, the Holy God, is the only one who can sanctify us. On the other hand, you encounter much of the New Testament's teaching and, for instance, you read Colossians 3 and Paul exhorts the believers to put to death, right? Put to death. This is the language of moral effort or spiritual effort. They have to do something. In 2 Peter, the author says God has given everything that we need to live a godly life in Christ Jesus. So, God has given us everything that we need, but then the exhortation, then, is add to your faith all these things culminating in love. And so, we can see that sanctification, this process of becoming more and more reflective of who Jesus is requires a "both/and." It's recognizing that God is at work in us to will and to act according to his good purpose, recognizing that we can only act according to his good purpose because God is at work in us, but at the same time, there's work. We need to work out our salvation with fear and trembling.