What does 'sanctification' mean in the Reformed tradition?

Sanctification, from a Reformed perspective, is the ongoing work of God in the life of a believer that enables them to grow in holiness and righteousness. As expressed in John 17:17, 'Sanctify them through thy truth: thy word is truth,' the word of God is a primary means through which the Holy Spirit works in believers’ lives. While justification is a one-time act of God declaring the sinner righteous, sanctification is a gradual process where the believer is transformed more into the image of Christ. It emphasizes reliance on the Holy Spirit to elicit change, rather than self-directed efforts, reinforcing the belief that apart from God, no good can come from us.