Not my book review, but found this one, by "Richard": 

This is truly one of the oddest books I've ever come across, in my long experience of odd books. It is written by Hauptmann who won the Nobel Prize for Literature in 1912 (before he wrote this) for his many plays. The book is about the adventures of a man who believes that he is a son of God and wanders through Germany giving testimony of Christ. I've never read anything like it although parts of it remind me of the Journal of George Fox, the founder of Quakerism and the sense of the book reminded me a little of Dreyer's movie "Ordet." But within literature, no - it's not really like Dostoyevsky's "The Idiot", although it would seems like it would be. 

While Quint is clearly insane (in a way that George Fox is not), the effect he has on people is quite powerful, and by far the most interesting part of the book is the way his simplistic (yet in a way profound) views of Christianity interact with things like socialism and more traditional Christianity. The author is unafraid to put his poor character in all sorts of difficult situations. The tremendous sympathy shown by the author for Quint is also surprising and greatly adds to the book - he is not mocked. There's also a powerful realism to the book, and interesting parallels between Quint's actions and those of the socialist reformers (referenced in Turgenev's "Virgin Soil") who used to wander the German countryside in a similar fashion.

The book also grows and changes on you - and Quint grows and changes - the real miracle of the book is how it retains its fascination and goes places you think it might be afraid to go. Its oddness also seems to increase rather than decline. By the end you feel exhausted by this endlessly weird book.

Apparently this book inspired a lot of discussion in its day, although it is rarely read now. I found this on the web which was helpful:

I strongly recommend the book, but only to those who are unafraid to read anything, as I can imagine a high percentage of readers giving up after the first few pages, perhaps feeling offended (why am I reading this?). I implore you however if you start to keep going, the book is not what it seems at first and becomes almost monstrous in its bizzareness, as Quint's following grows, and his followers become more and more totally off the wall. Familiarity with Christianity is helpful but not required. One must understand when reading however that the book is not unrealistic, at that time of its writing this type of thing was not inconceivable. Heck crazy things like Mormonism (as the book points out), Christian Science, and Seventh Day Adventism began in the United States at the time. While the book is German, the Herrnhuter (mentioned by Goethe in Young Werther) - also known as the Morvian Brotherhood - a Christian "cult" dating from the Middle Ages - is part of the book. Nothing remotely like this is being written today - the tone of the book especially would not pass muster.