The religious world is but the reflex of the real world. And for a society based upon the production of commodities, in which the producers in general enter into social relations with one another by treating their products as commodities and values, whereby they reduce their individual private labour to the standard of homogeneous human labour – for such a society, Christianity with its cultus of abstract man, more especially in its bourgeois developments, Protestantism, Deism, &c., is the most fitting form of religion.
Thinking back on this banger from the first pages of Capital. In order to make full sense of this, pair it with this other related quote from the first German edition:
It is as if alongside and external to lions, tigers, rabbits, and all other actual animals, which form when grouped together the various kinds, species, subspecies, families etc. of the animal kingdom, there existed also in addition the Animal, the individual incarnation of the entire animal kingdom. Such a particular which contains within itself all really present species of the same entity is a universal (like animal, god, etc.). Just as linen consequently became an individual Equivalent by the fact that one other commodity related itself to it as form of appearance of value, that is the way linen becomes – as the form of appearance of value common to all commodities – the universal Equivalent universal value-body, universal materialization of abstract human labour. The specific labour materialized in it now thereby counts as universal form of realization of human labour, as universal labour.
Drawing from Rubin here: The private labor of an individual producer becomes social labor only when it is assimilated with an actual other commodity that represents all commodities, as the universal commodity i.e. money.
It is in this sense that Christianity is the most fitting religion for capitalist society. Capitalism produces money which stands in as the concrete form of an abstraction. This is just as absurd as if one could observe an animal walking around which is actually the Animal in the abstract.
Christianity has an analogous form in the role of Jesus. Jesus, representing the abstract human, has to take on the concrete form of a human being, in the flesh, in order to represent humanity. The debt owed by each human being (original sin) is paid in the form of the abstract universal, money Jesus whose death is supposed to actually consummate the payment.