The theme of religious belief being an obstacle to progress is a common one. Below, a boy entranced by the vision of the future struggles to get away from his grandmother as she drags him to a shadowy church. In the Soviet Union it was often grandmothers who baptized children and taught them to pray, and here that is depicted as a sort of child abuse.
The piece below is my favorite: the theme is similar but with a much better composition: the Orthodox cross serves as prison bars for the emaciated believer who could just step outside and join the bright, joyous, healthy, real world. It underlines the tragedy of religious belief, as the believer could simply decide to leave his belief behind and become a real man.
Religious believers are usually portrayed as poor, elderly and backwards, as below:
On the left a poor elderly couple prays in the darkness. On the right a handsome young couple participates in the light of Soviet society.
The power of the propaganda is twofold. First, it illustrates the Marxist critique of religion as an opiate of the people and enemy of progress. Only by abandoning faith can one be free to create a just and prosperous future. In that sense these posters are not that different from the contemporary atheist portrayal of belief as the obstacle of progress and unbelief as the liberation of man.
Second, there is the implicit threat of marginalization: in the Soviet Union participating in a conspicuous religious practice such as attending Mass was an obstacle to party membership and a sure way to find yourself locked out of social and economic opportunity.
Ironically, with historical hindsight we can read the religious characters above in the exact opposite way that the artists intended, as heroes refusing to participate in a lie at the cost of their own marginalization.