When the Holy Spirit has worked and accomplished this, and a person’s will has been changed and renewed by His divine power and working alone, then the new will of that person is an instrument and organ of God the Holy Spirit. So that person not only accepts grace, but he also co-operates with the Holy Spirit in the works that follow.
From the Formula of Concord, Epitome II.18.
That lovely phrase, “the new will of that person is an instrument and organ of God the Holy Spirit” wonderfully sums up the relationship between the work of the Holy Spirit and the regenerate will of the believer.
This is why St. Paul was able to write to the Romans, “… be transformed by the renewal of your mind” (Rom 12:2 ESV). In other words, be what you have been made, and become what you are.