Many people will be surprised to discover that the work of the Holy Spirit was not developed as a doctrine until after the Protestant Reformation. Of course, Jesus taught His disciples about it, the letters of the apostles were filled with it and, if Pentecost means anything, the church could not have existed without it. People experienced the Spirit in their hearts but, when they talked about theology, it was usually about something else – could the God of the Old Testament be equated with the Father of Jesus Christ? Was the Creator God also the Redeemer? How can Jesus Christ be divine when there is only one God who is transcendent and completely different from anything He has made? These were the questions that were debated, and it was only late in the day that attention turned to the work of the Third Person of the Trinity as distinct from that of the other two.
From: God Has Spoken: A History of Christian Theology by Gerald Bray (Wheaton: Crossway, 2014), p. 723.