Full Idea
Law, morality, religion are to the proletarian so many bourgeois prejudices, behind which lurk in ambush just as many bourgeois interests.
Clarification
'Proletarians' are working class, 'bourgeois' are middle class
Gist of Idea
For the proletariate, law, morality and religion are just expressions of bourgeois interests
Source
K Marx / F Engels (The Communist Manifesto [1848], §1)
Book Reference
Marx,K./Engels,F.: 'The Communist Manifesto', ed/tr. Taylor,A.J.P. [Penguin 1973], p.92
A Reaction
This Marxist idea has been the main driving force in spreading relativist views through modern culture. There is a lot of truth here, but philosophy is plausibly the search for truths which both the bourgeois and the proletarian will accept.