When I studied church history in seminary, the text that was assigned was The Story of Christianity: Volume 1 The Early Church to the Dawn of the Reformation by Justo L. González. I just recently reread this book as a part of my ongoing research.
I am reminded of what a tremendous resource this is. González is a tremendous scholar and he is equally a gifted writer. Church history can easily become a bog of names and dates that leaves the reader wanting to throw the book against the wall. But that is not the case with González.
González is able to get across the big story that provides the reader with everything they need to know but in a readable and accessible form. He traces out the major themes and introduces the important characters and yet it is somehow not overwhelming.
This first volume takes us from the beginning of Christianity to just before the reformation. That is quite the task, covering many centuries and seeing radical changes in the church. González is able to navigate these events by breaking down the history into sections such as: the early church, the imperial church, medieval Christianity and the beginnings of colonial Christianity.
There are more comprehensive church histories out there, but this is the introduction to church history that I recommend. It is the perfect place to start and to discover the eras that you want to learn more about.