This piece on rethinking Marx and religion is intriguing work. In addition to his well-known "opiate of the masses" remark from the introduction to Contribution to the Critique of Hegel's Philosophy of Right (1844), Marx made several other remarks on religion:
The Established Church ... will more readily pardon an attack on thirty-eight of its thirty-nine articles than on one thirty-ninth of its income. Now atheism itself is a culpa levis [a venial sin], as compared with the criticism of existing property relations. [Preface to Capital, Vol. 1 (1867)]
Christianity is...the special religion of capital. [...] One man in the abstract is worth just as much or as little as the next man. In the one case, all depends on whether or not he has faith, in the other, on whether or not he has credit.
[Theories of Surplus-Value, Chapter 24 (1863)]
Paul Lafargue's The Religion of Capital [here or here (PDF)] (1887) and Walter Benjamin's Capitalism as Religion (1921) are also intriguing, perhaps eventually leading to John Raines' Marx on Religion.