Take for instance the ontological proof [of god]: "that which I conceive for myself in a real way is a real concept for me", something that works on me. In this sense, all gods, the Pagan as well as the Christian ones, have possessed a real existence...If somebody imagines he has a hundred thalers...he will incur debts on the strength of his imagination
(Marx, "The Difference Between the Democritean and Epicurean Philosophy of Nature", 1841)
And as for Rome! Read Cicero! The Epicurean, Stoic or Sceptic philosophies were the religions of cultured Romans when Rome had reached the zenith of its development. That with the downfall of the ancient states their religions also disappeared requires no further explanation, for the "true religion" of the ancients was the cult of "their nationality", of their "state".
(Marx, "[On the] Leading Article in No. 179 of Kölnische Zeitung, July 10 1842")
Up till now the political consitution has been the religious sphere, the religion of national life, the Heaven of its generality over against the Earthly existence of its activity.
(Marx, "Contribution to the Critique of Hegel's Philosophy of Law", 1843 Marx's emphasis)
Man, even if he proclaims himself an atheist through the medium of the state, that is, if he proclaims the state to be atheistic, still remains in the grip of religion, precisely because he acknowledges himself only by a roundabout route, only through an intermediary. Religion in precisely the recognition of man in a roundabout way.
(Marx, "On The Jewish Question", 1843 my emphasis)
[In modern society religion] is no longer the essence of community, but the essence of difference...[Religion becomes] only the abstract avowal of specific perversity, private whims and arbitrariness.
(Marx, "On The Jewish Question, 1843 Marx's emphasis)
Religion is the general theory of [the world of class society], its encyclopaedic compendium, its logic in a popular form, its spiritualistic point of honour, its enthusiasm, its moral sanction, its solemn complement, its universal source of consolation and justification. It is the fantastic realisation of the human essence because the human essence has no true reality...Religion is the sigh of the oppressed creature, the heart of a heartless world...It is the opium of the people.
(Marx, "Introduction to a Contribution to the Critique of Hegel's Philosophy of Law")
Young Marx definitely would agree. Let's jump over to Capital!Marx
In order, therefore, to find an analogy [for commodity-fetishism] we must take flight into the misty realm of religion. There the products of the human brain appear as autonomous figures endowed with a life of their own, which enter into relations both with each other and with the human race. So it is in the world of commodities with the products of men's hands. I call this the fetishism which attaches itself to the products of labour as soon as they are produced as commodities, and is therefore inseparable from the production of commodities.
Yeah, he'd agree too.