How do we define Modernism and Postmodernism? The citations below, from Dr. Richard Winter, were very helpful to me in defining these unfamiliar and complex philosophical terms:

Dr. Richard Winter, M.D. Professor Emeritus of Counseling at Covenant Theological Seminary

In our culture we live at the confluence of two highly influential rivers of thought: modernism and postmodernism. These are the humanist and existential ideas that, over the last three hundred years, have replaced Christianity as the dominant world view in our culture. They flow through and around us, affecting everything, often without being named or recognized, shaping both our way of thinking about ourselves and the world we live in—for good or ill. 

— Richard Winter, Perfecting Ourselves to Death, 149.

Here are the definitions that made plain the meanings of Modernism and Postmodernism for me. Stating it bluntly, these simple explanations “cleared the fog” of uncertainty! My exposure to philosophy has been minimal. I’ve picked-up a few concepts here and there in studying theology and church history. Philosophy, however, is an entirely different discipline that has its own set of academic terms and its own catalogue of philosophical jargon. I hope Dr. Winter’s explanations can be as helpful to you as they were to me.

Put very briefly, modernism is that belief that with science, reason and technology we can make our world a better place. Our identity from a modernist perspective, is defined by the objective, scientific experts of genetics, anthropology, psychology, sociology, biology and other sciences. Reality is what is described and measured. Biologists or sociologists, for example, tend to reduce the fundamentals of human nature to biological or sociological processes, respectively. God is no longer needed (in this modernist perspective) as part of the explanation of how things work or to give a moral framework for living. 

However, even from the beginning of modernism, in what we call the Enlightenment, there was a reaction against this emerging mechanistic view of human nature. In an attempt to preserve the significance and dignity of persons in a scientific age, these reactionary movements—seen in romanticism, existentialism and mysticism—emphasized personal and subjective values almost to the exclusion of the mechanistic and objective. The pendulum swung to the other extreme. It was these ideas that became what we now call postmodernism, in which, without a reference point outside of ourselves in revelation, reason or science, we are left only with subjective experience and freedom to choose to be whatever we want to be—to invent ourselves. Self-fulfillment becomes a core value. 

— Richard Winter, Perfecting Ourselves to Death, 149-150.

As you encounter these two terms in conversation, literature, film, and music, you now have an explanation that makes their meaning plain. It is interesting to note that in both Modernism and Postmodernism the need for a transcendent God is steadfastly rejected. The Modernists deny God’s very existence by insisting that they are the masters of their own destiny. And the Postmodernists elevate the Self to the position of the one who determines all reality and truth. In this sense, as Christians, we see the utter futility of these two philosophical systems. Yet, it benefits us to understand the philosophical “world views” of those whom God brings into our life. How else can we best know how to answer the questions they pose in response to the Gospel? Therefore, it helps us to understand where people are coming from when we seek to “make a defense to anyone who asks…for the hope that is in [us].” (1 Pet. 3:15)

— Dr. Marcus J. Serven

Source: Richard Winter, Perfecting Ourselves to Death: The Pursuit of Excellence and the Perils of Perfectionism, Downers Grove, IL: InterVarsity Press, 2005. Pages 149-151