I am a follower of Marxism, and Kautsky is definitely NOT a Marxist. His brand of "socialism" has been long dead, as is Bernstein's, Stalin's, Bakunin's, Lasalle's and Proudhoun's and I would be shocked if ANYONE this side of a university would know any of these people (with the possible exception of Stalin) or care to know any of them or what they stood for. Even people that speak in their name (probably only college Anarchists for Bakunin or Proudhoun) probably have little concept of what they stood for and would more than likely oppose them if they knew (Bakunin was for an "elite group" of "revolutionaries" controlling the economy and Proudhoun was against technology and not really against capitalism unless it was "unfair"). Kautsky's politics were a shade pinker than Bernstein's, and more "statist" than the Spanish or Russian anarchists, but they all fell in the same camp against the Bolsheviks, along with the foreign invaders (the U.S. among them), the proto-fascists (the "Black Hundreds" as they were called"), the other patriotic "socialists", and the Czarists. Distinguishing between these socialist fakers may have been interesting at the time for some intellectual pedant, but today they all blur together and are utterly irrelevant.
Having said that, his work on Christianity is brilliant. Some of the facts in this book are outdated (it was written in 1909 after all), but most of it is dead on. Yes, some of it is conjectural and hypothetical, mainly because of the massive falsification of the early and proto-Catholic church, but the purely theoretical aspect is great and proven to be true more and more with further discoveries into ancient civilization.
The key reason I recommend this book is because of it's CLASS ANALYSIS. Anyone that reads the Bible will come across the ENORMOUS amount of material in it that directly relates to the class situation the characters in it were in. Nearly all Bible scholars (the religious ones primarily, of course) generally skip over this and you end up with a near total lack of understanding about what the Bible represented in it's various sections, or what a "Christian" even means. From the rich Jews imperialism of the early Bible to the impoverished early communists of an anti-Roman revolutionary sect of Jews known as Christianity, the KEY to the Bible is the class struggle, not God, morality or faith.
Anyone who wants the truth on the heretical Jews that called themselves "Christians" should read this book.