Freud starts from a Hobbesian vision of the relationship between Nature and Culture:
It is remarkable that, little as men are able to exist in isolation, they should nevertheless feel as a heavy burden the sacrifices which civilization expects of them in order to make a communal life possible. Thus civilization has to be defended against the individual ... (page 2)Not only that, but this business of defence requires two classes of citizens, rulers and ruled:
It is just as impossible to do without control of the mass [Masse] by a minority as it is to dispense with coercion in the work of civilization. For masses are lazy and unintelligent ... (page 3)At this point, the bells of political correctness would begin to ring in the ears of a modern editor. Thank you, Dr Freud, but this is not for us.
Get past this, and quite soon Freud is arguing that religious ideas have arisen "from the necessity of defending oneself [psychologically] against the crushingly superior force of nature" (page 17) though this narrow basis is later expanded to include other experiences of helplessness:
the terrifying impression of helplessness in childhood arouses the need for protection - for protection through love which was provided by the father; and the recognition that this helplessness lasts throughout life made it necessary to cling to the existence of a father ... Thus the benevolent rule of a divine Providence allays our fear of the dangers of life; the establishment of a moral world-order ensures the fulfilment of the demands of justice, which have so often remained unfulfilled in human civilization .... (page 26)But since wish-fulfilment of this kind plays such a prominent role in the origin of religious ideas - which are, in their nature, not susceptible to proof or disproof - then they are properly called illusions. Hence the title of the book.
And Freud thinks we would be better off without them, trying to accept our helplessness but, at the same time, realising that scientific progress can do much to mitigate it. It is simply a more dignified way of living.
There is a separate line of argument which emerges at the end: religion is actively bad for us:
Think of the depressing contrast between the radiant intelligence of a healthy child and the feeble intellectual powers of the average adult. Can we be quite certain that it is not precisely religious education which bears a large share of the blame for this relative atrophy? ...Is it not true that the two main points in the programme for the education of children to-day are retardation of sexual development and premature religious influence (page 43)
As Lenin once said of his opponents, Don't listen to what they are saying; watch what their hands are doing.