"Marx-the Communist Manifesto and enough writings on Communism to fill a small library."
Marxism is not a religion, it is a socio/economic/politico/ theory, and to describe it otherwise is a typical lie of those who would continue with the practice of the enforced teaching of children by men and women of doubtful repute.
In the "small library's" worth of Marxist literature, religion features very small.      
The oft-quoted "opium of the people" was intended for the introduction of a work by Marx, 'A Contribution to the Critique of Hegel's Philosophy of Right'; which was actually never written - the introduction alone was published in 1844 in Marx's own journal.
It is an continuing lie claim Marxism as "a religion", and it is a total distortion of the facts to say that Marx or any major writer devoted anything but a passing mention to religion other than as a tool of oppression.
On the other hand, the last Tzar of Russia, at the height of his power was heavily influenced by an insane Christian monk who is regarded by many in modern non-Communist Russia as saintly.

"Grigori Yefimovich Rasputin (Russian: Григорий Ефимович Распутин; IPA: [ɡrʲɪˈɡorʲɪj jɪˈfʲiməvʲɪtɕ rɐˈsputʲɪn]) (22 January [O.S. 10 January] 1869 – 29 or 30 December [O.S. 16 December] 1916) was a Russian mystic and advisor to the Romanovs, the Russian Imperial family. He was never officially connected to the Orthodox Church but considered a "strannik" (or pilgrim) wandering from cloister to cloister. He is even regarded as a starets (ста́рец, "elder", a title usually reserved for monk-confessors), believing him to be a psychic and faith healer.[1] He impressed many people with his knowledge and ability to explain the Bible in an uncomplicated way.
In 1907 Rasputin was invited for the first time by Tsar Nicholas II and Alexandra as a healer for their only son, Tsarevich Alexei, who suffered from hemophilia. It is supposed he became an influential figure in the later years of the Tsar's reign, especially after September 1915. It has been argued[2] that Rasputin helped to discredit the tsarist family, leading to the fall of the Romanov dynasty in February 1917. The Tsarina and her family saw Rasputin variously as a saintly mystic, visionary, healer and prophet but his enemies, as a debauched religious charlatan, heavily interested in sexual relations with his followers. There has been much uncertainty over Rasputin's life and influence, as accounts have often been based on dubious memoirs, hearsay and legend.[In Russia he is nowadays seen by many people and clerics, among them Elder Nikolay Guryanov as a righteous man"
I thought Christians were "seekers after truth - tut-tut
Jim Carroll

Some facts about Marxism and religion from, a pamphlet I picked up when I was working as an electrician at the Humanist Society headquarters in London.

"The founder and primary theorist of Marxism, the nineteenth century German sociologist Karl Marx, had an ambivalent attitude to religion, viewing it primarily as "the opium of the people" that had been used by the ruling classes to give the working classes false hope for millennia, while at the same time recognizing it as a form of protest by the working classes against their poor economic conditions. In the end, Marx rejects religion.
In the Marxist-Leninist interpretation of Marxist theory, developed primarily by Russian revolutionary Vladimir Lenin, religion is seen as negative to human development, and socialist states that follow a Marxist-Leninist variant are atheistic and explicitly antireligious. Due to this, a number of Marxist-Leninist governments in the twentieth century, such as the Soviet Union and the People's Republic of China, implemented rules introducing state atheism. However, several religious communist groups exist, and Christian communism was important in the early development of communism."