Of the many critical concepts that emerge from the Marx/ Engels work, I think that the idea of viewing historical consciousness as a reality of socio- economic construction is vital to understanding the work.  Marx/ Engels are truly seeking to develop a new frame of reference in how people construct and view reality.  In a setting where industrialist- based capitalism has become so dominant in the time period, to develop a frame of reference that argues that the materialist dynamic is the dominant mode of viewing being in the world is critical to the work's understanding.  The idea of dialectical materialism is the central tenet of the work.  Accept this ide and Marx/ Engels almost seem to be suggesting that the embrace of socialism/ communism as a means of supplanting capitalism is almost a guarantee.  For Marx/ Engels, it is vitally important that the proletariat and working class see their condition as one of exploitation at the hands of the economic elite who need this dialectic in order to substantiate their own conditions.  It seems to me that this becomes the critical element because all change that is being advocated springs from this foundational tenet of economics being the structure of all.