"In our time, when most Christians view the Eucharist as a means of achieving unity in the church, Elert's history and analysis in this book demonstrates that for the early church, the opposite was true: the Eucharist was a fruit of unity, and those who were not united in teaching and worship did not commune together.
While this book is dense and insightful, it's not inaccessible to the lay reader. There are frequent Greek references, but it's not essential to know Greek (I have only a passing knowledge of the language) to benefit.
Finally, the most dense analyses ('Communio in Early Church Usage,' 'The Origin of the Formula Sanctorum Communio,' and 'Koinonia and the Holy Things') are covered in three respective excursuses in the back of the book.
I think that Elert's Lutheranism sometimes gives a slant that is perhaps unwarranted. (I'm especially thinking of his analysis of the "3 walls" guarding the early church fellowship -- bishops, the canon of scripture, and the regula fidei, the creed of the church -- , and how a succession of bishops is given the most critical treatment of the 3).
But what tiny misgivings I might have are dwarfed by the great things about this book. It's one of those treasure mines that are worth digging in anew every few years. Like any great book, this one challenges the way we think of ourselves as Christians, and the way that thinking plays out in the lives of the church."