Initial sanctification occurs instantly at the moment of salvation when we are delivered from the kingdom of darkness and brought into the Kingdom of Christ (see Colossians 1:13). Progressive sanctification continues over time until we go to be with the Lord. Initial sanctification is entirely the work of God the Holy Spirit who imparts to us the very life of Christ. Progressive sanctification is also the work of the Holy Spirit, but it involves a response on our part so that we as believers are actively involved in the process.
