Foundations of Christianity (German: Der Ursprung des Christentums) is a 1908 book by Marxist theoretician Karl Kautsky.

The first edition in English, published in 1925 by George Allen and Unwin, followed the eleventh German version.

In his foreword, Kautsky expressed his hope that the book would be 'a powerful weapon in the struggles of the present, in order to hasten the attainment of a better future'.

He began his analysis by looking for evidence that 'the person of Jesus' existed at all, using pagan and Christian sources.

The next dozen chapters are then taken up with a materialist description of the ancient Roman society from which early Christianity sprang.