Historical Theology: An Introduction to Christian Doctrine

Alister McGrath’s Historical Theology: An Introduction to the History of Christian Thought is probably what you are looking for. Though he is an Anglican theologian, he writes his historical textbooks with the detached objectivity of a historian. He doesn't try to prove who was "right"; he simply explains why they thought what they thought based on the cultural and political pressures of their time. He presents the arguments of both sides using their own internal logic. He includes "case studies" at the end of each era where you look directly at primary source texts without heavy-handed commentary. I'll give a shout-out to Gregg Allison's Historical Theology: An Introduction to Christian Doctrine. Though it's written from an evangelical perspective, its main advantage is written as a companion to Wayne Grudem’s Systematic Theology. So it's great if you want to understand exactly how a single, specific doctrine (like the Trinity or Justification) evolved over 2,000 years, from the Early Church to the modern era, without having to wade through any other doctrines.