Karl Marx founded Marxism, which is widely known as an influential political and economic system, but which is also a religion from a sociological perspective. Marx founded "Marxist Communism" and he was one of history's most important proponents of Communism. Marx co-authored The Communist Manifesto, which is probably the most important document in the history of Communsim, essentially the "scripture" or "holy book" of the movement. But Marx himself did not create Communism; it was an existing, organized philosophy that he was attracted to.
The passage below briefly describes the religious background and religious affiliation of Karl Marx.
Source: Josh McDowell & Don Stewart. Understanding Secular Religions. Here's Life Publishers, Inc.: San Bernardino (1982). Pages 45-46.
The name of Karl Marx is probably the best known of any founder of a political or economic system...From: Michael H. Hart, The 100: A Ranking of the Most Influential Persons in History, Hart Publishing Company, New York City (1978), pages 91-92:
Karl Marx was born in Trier, an ancient German city in the Rhineland... His ancestors, Jewish on both his mother's and father's sides, were rabbis. His father, Heinrich, had converted to Protestantism in 1816 or 1817 in order to continue practicing law after the Prussian edict denying Jews to the bar. Karl was born in 1818 and baptized in 1824, but his mother, Henriette, did not convert until 1825, when Karl was 7. While the family did not appear religious at all -- it was said that not a single volume on religion or theology was in Heinrich's modest library -- Karl was raised in an atmosphere of religious toleration. There was some discrimination against Jews in the area, but general religious tolerance was the standard. Karl was sent to religious school primarily for academic rather than religious training. On the whole, the family was not committed to either evangelical Protestantism or evangelical Judaism. Vincent Miceli notes:The family lived as very liberal Protestants, that is, without any profound religious beliefs. Thus, Karl grew up without an inhibiting consciousness of himself as being Jewish. In changing his credal allegience, or course, the father, newly baptized Heinrich, experienced the alienation of turning his back on his religious family and traditions. Thus, though politically emancipated and socially liberated from the ghetto, the experience of being uprooted and not completely at home in the Germany of the nineteenth century did affect the Marx family (Miceli, Atheism, pp. 94, 95).
Marx's writings form the theoretical basis of Communism, and in view of the extraordinary growth of that movement in the twentieth century, it is clear that he deserves a high place on this list [of the 100 most influential persons in world history]. The question is how high should he be ranked?
A major factor in that decision is one's estimate of the importance of Communism in the long-term history of the world. Since the rise of Communism is a part of recent history, it is hard to get an accurate perspective. However, though one cannot be sure just how far Communism will go and just how long it will last, it should be apparent by now that the ideology is solidly entrenched, and will be a major influence in the world for at least a few centuries to come.
At the present time , about one century after Marx's death, the number of persons who adhere at least nominally to Marxism is close to 1.3 billion. This is a greater number of adherents than any other ideology has had in the entire history of mankind--not only in absolute numbers, but also as a fraction of the total world population. This has led many Communists, and some non-Communists as well, to believe that the future may see the eventual worldwide triumph of Marxism. It is very difficult, though, to extrapolate such trends into the future with confidence. There have been many ideologies which seemed very important during their heyday, but which eventually died out. (The religion founded by Mani is an interesting example.) Back in 1900, it seemed obvious that constitutional democracy was the wave of the future. Hopefully, that will eventually be the case, but few would say any longer that such an outcome is assured.