The Spirit is the one working faith in believers, the faith that is the basis for this gratitude. Therefore, Christians cannot take credit for the improvements in their lives; the source and cause is still God's grace. At the same time, this progression is not a mysterious process. It is in faith that one responds in gratitude, and the good works that follow are responses of thankfulness rather than attempts to appease a God who is keeping score.
Luther was convinced that a religious message that did not proclaim the complete forgiveness of sins without any human work or merit could not produce the genuine and free acts of love that come from believing the gospel. Only when a believer has been released from any threat of punishment can there be genuine sanctification. When the fear of doing too little is taken away, as well as the pride of assisting in one's salvation, the believer begins to live a life of genuine love.
"Luther and the Holy Spirit: why pneumatology still matters" Currents in Theology and Mission , April, 2007 by Jeffrey K. Mann