The What is religion? page of this site gives a very brief outline of postmodernity. This page will provide you with a little bit more detail. Remember, the OCR specification requires that you consider the challenge that postmodernity presented to Christianity and also the ways in which Feuerbach, Smart and Cupitt respond to that challenge.
Postmodernism developed as both a continuation of and a reaction to modernity. It emerged in the early years of the twentieth century and reflects a loss of faith in the beliefs and certainties of modernity.
For example, postmodernists questioned the idea that there was an objective reality out there to be discovered and and they abandoned foundationalism (the idea that we can identify a sound foundation upon which to build other knowledge). Furthermore they were suspicious of the type of metanarratives tjat modernists sought (grand accounts which were supposedly true for everyone). Postmodernists questioned the concept of objective truth and believed that everything is subjective and relative. Consequently, although they did not reject the use of reason in certain areas they did not believe it is the only or even the main way to investigate the world. Emotional responses were given equal status to logical ones. Many postmodernists questioned modernity's faith in science and technology to lead to progress and some were very suspicious about the dangers of technology.
To a certain extent postmodernism developed out of the failings of modernity to come to a complete understanding of the world. Various developments in science (such as the discovery of quantum physics) made it clear that the world was significantly more complicated (and thus much more difficult to understand) than many modernists had thought. The world wars of the early twentieth century demonstrated the potential dangers of technology and made the earlier optimism that the world was improving seem idealistic and misplaced.
Encyclopaedia Britannica Postmodernism page.