The basis of irreligious criticism is: Man makes religion, religion does not make man. In other words, religion is the self-consciousness and self-feeling of man who has either not yet found himself or has already lost himself again. Man is the world of man, the state, society. This state, this society, produce religion, a reversed world-conscience, because they are a reversed world. Religion is the general theory of that world, its encyclopaedic compendium, its logic in a popular form, its spiritualistic point d’honneur, its enthusiasm, its moral sanction, its solemn completion, its universal ground for consolation and justification. It is the fantastic realization of the human essence because the human essence has no true reality. The struggle against religion is therefore mediately the fight against the other world, of which religion is the spiritual aroma.
Religious distress is at the same time the expression of real distress and the protest against real distress. Religion is the sigh of the oppressed creature, the heart if a heartless world, just as it is the spirit of a spiritless situation. It is the opium of the people.
The abolition of religion as the illusory happiness of the people is required to their real happiness. The demand to give up the illusions about its condition is the demand to give up a condition with needs illusions. The criticism of religion is therefore in embryo the criticism of the vale of woe, the halo of which is religion. (page 64-65)
Beside that introduction about religion, Marx then jumps on his hobby horse about what the future German state could be: if only we didn't have any private property or classes and the state controlled everything. (What is Marx without a struggle and a revolution?) He also is a little bit positive about the Reformation, although he thinks it didn't quite go far enough:
For Germany’s revolutionary past is theoretical, it is the Reformation. As the revolution then began in the brain of the monk, so now it begins in the brain of the philosopher.
Luther, we grant, overcame bondage out of devotion by replacing it by bondage out of conviction. He shattered faith in authority because he restored the authority of faith. He turned priests into laymen because he turned laymen into priests. He freed man from outer religiosity because he made religiosity the inner man. He freed the body from chains because he enchained the heart. (page 70)
Marx's main gripe with religion is that it is man made. He draws a contrast between devotion and conviction and says that it has enchained the heart. Might it not be possible that all men are enchained by something, and that something 9 time out of 10 be man made? Joshua appealed to his nation to chose what gods they would serve. Jesus dissed the greedy religious leaders of his time telling them that they have to chose what they devote themselves to. Paul says that we are all slaves to something, we just need to choose what we want to follow.
People are hugely devoted and convicted towards all sorts of things like sport, TV shows, food, exercise, pleasure, social reform etc... We are all wired to devote ourselves to something, weather it be ourselves, or some cause that is bigger than ourselves. Religion isn't the only man made thing that weighs man down.
Marx sees that once we make man to be the highest being we can then be free:
The only practically possible liberation of Germany is liberation from the point of view of the theory which proclaims man to be the highest essence of man. In Germany emancipation from the Middle Ages is possible only as emancipation from the partial victories over the Middle Ages as well. In Germany, no kind of bondage can be shattered without every kind of bondage being shattered. The fundamental Germany cannot revolutionize without revolutionizing from the foundation. The emancipation of the German is the emancipation of man.... (page 74)
I'm not sure what we would be free from as we would still be serving some one, in this case man. Some men are lazy and not actually a good representation of the "highest essence of man." I wonder what model of a man did Marx have in mind? Ayrans? Semitic? People from all countries or a German man? A man from the under class or someone from the upper class? If only we could fine in history a perfect man who could be seen as the "highest essence of man". Someone who would actually free us from something big like death and guilt. This saviour of man would certainly be worth following.
You can read a slightly different translation than the one in the Portable Atheist of Marx's Introduction to A Contribution to the Critique of Hegel’s Philosophy of Right here.