What do you mean by the term “paradox” in the book’s title?

There are several questions people ask me about my latest book, The Paradox of Church and World: Selected Writings of H. Richard Niebuhr.  Here is one of them.  

Question: What do you mean by the term “paradox” in the book’s title?  
 
My answer: The “paradox” refers to the longstanding notion that the church must always be “in” but not “of” the world. One sees it already in the High Priestly Prayer of Jesus in John 17, and the concept is paradoxical because it highlights two accurate, but seemly contradictory, ideas, which must be kept in tension with each other if the church in any age is to honor the calling and mission that Jesus as its Lord has given to it.  
 
“Ultimately,” or so H. Richard Niebuhr wrote as early as 1929, “the problem of church and world involves us in a paradox; unless the church accommodates itself to the world, it becomes sterile inwardly and outwardly; unless it transcends the world, it becomes indistinquishable from the world and loses its effectiveness no less surely.”  This same “paradox” became one with which he would wrestle and from which he would gain a host of valuable insights throughout the remainder of his career as one of America’s most respected Protestant theologians.