Role of the Holy Spirit

What is the role of the Holy Spirit in the life of a Christian?

As true God along with God the Father and God the Son (Jesus Christ), the Holy Spirit participates in the divine works that the Father and Son also do. This would include giving and maintaining our physical lives (creation and providence work) and convicting us of our sin, guilt, and need for the Savior’s work so that we may grasp the redeeming work of Jesus through Spirit-given faith. And he in a miraculous way guided the human writers (prophets and apostles) of the Bible books to write precisely what God wanted them to provide for our faith-life.

You ask more specifically what the Holy Spirit does for believers. Luther’s famous Explanation of the Third Article of the Apostles Creed offers an excellent summary:

I believe that I cannot by my own thinking or choosing believe in Jesus Christ, my Lord, or come to Him. But the Holy Spirit has called me by the Gospel, enlightened me with His gifts, sanctified and kept me in the true faith. In the same way He calls, gathers, enlightens, and sanctifies the whole Christian Church on earth, and keeps it with Jesus Christ in the one true faith. In this Christian Church He daily and fully forgives all sins to me and all believers. On the Last Day, He will raise me and all the dead and give eternal life to me and all believers in Christ. This is most certainly true.

You also ask if the Holy Spirit is really necessary for spiritual growth and positive changes in our spiritual lives — or whether sanctified common sense is enough. The Spirit is necessary if we are talking about true spiritual growth, changes in our heart and inner attitudes and motives — in short, more than external behavior changes. The power of God is needed for such transformations in faith and the resulting faith-life. We can only change external behavior patterns on our own — which may look good on the outside for a while, but will lack the inner changes in motives and attitude that will characterize the real deal.

For more detailed information, with Bible passages that speak of these things specifically, ask your pastor for a book that will provide that kind of information.