Postmodernism inherently flows out of, or is a reaction against, modernism. To begin to understand what postmodernism is, we must first examine (briefly) its predecessor.
Modernism is a philosophical and ideological movement which blossomed a century ago. Many cite the atrocities of modern war in the first World War as the catalyst for this movement, which led many to reject religion and focus instead on human individuality and progress. Thought turned introspective, examining the self and the anthropomorphic roadblocks to bettering society. Much of this thought was prevalent in art and literature, though it trickled into all aspects life, including theology.
Postmodernism, too, has a strong relationship to art and thought, though contra to modernism, it rejects the “modernist, avant garde, passion for the new.” Much of postmodernism is concerned with architecture and literary criticism, though here too theology and ecclesiology is strongly affected.
It tends toward religious pluralism (all religions are equally valid) and relativism—perhaps modern relativism was even born out of this movement. Thus a postmodernist outlook views Christian morality as good for Christians, but not necessary for others of different belief systems. Where modernism draws lines around the “self” and focuses on the individual, postmodernism blurs those lines, making all things acceptable morally and ethically. A “sarcastic playful parody of western modernity and the “John Wayne” individual and a radical, anarchist rejection of all attempts to define, reify or re-present the human subject” (Keep, McLaughlin, Parmar, “Defining Postmodernism“).
Citing postmodernism as good or bad may be unhelpful. Instead, I pose a question: what is shaping your worldview? Do you look to the trends and thought movements of society to shape your concepts of truth, or of right and wrong, or do you look to Scripture? Do you believe in The Absolute Truth, or do you think yours is one opinion among many?
As the Church, either our foundational worldview will be strengthened by the true and trustworthy Word of God, or it will be weakened by the word of the world. We must, through the Spirit, be the always being reformed church, being built and strengthened in the image of God, not society.
(A long but helpful article on postmodernism can be found in the Encyclopedia of Christianity, Volume 4)