Today we have multiple sources of theology written out for us, doctrines formulated and supporting verses compiled. But the early Christians didn’t have a written out theology yet. They had the Word of God, from which we get our theology, but it wasn’t neatly put together or defined yet. One of the benefits that we can gain from studying theology is to look at the history of how theology came about or how the particular doctrines that we hold to today were defined and articulated in the early church. The Trinity isn’t specifically mentioned in the Bible and this doctrine was hashed out in the early church and defined so that today we have a definition of the Trinity (even if we don’t fully understand it). Early heresies arose in the church over Jesus’ humanity and deity. Church councils were formed and creeds written to define theological truths that lined up with Scripture to refute these heresies. It can be a fascinating study to look at how these different doctrines were developed through the years of church history.
Church history books will touch on the subject of the history of theology, but for a more in-depth look, there are books that actually give the history of how theology came about. While not having read this fully myself, Gregg Allison’s Historical Theology is one of these. A long but detailed and fascinating book on the history of various doctrines is Gerald Bray’s God Has Spoken: A History of Christian Theology. There are others as well. In your study of theology, it can be a fun detour to study the history of the different doctrines, particularly on the doctrines of God the Father, Jesus, and the Holy Spirit.
William Cunningham’s Historical Theology (partial)