(Eph 1 - 2 Thess 3)
Finishing up the letters of Paul addressed to churches, one of the things I see that all of them have in common is a direct relationship between theology and practice. This is perhaps most apparent in the letter to the Romans, sometimes referred to as "The Gospel according to Paul." The first 11 chapters contain some of the deepest systematic theology in all of the new testament. But then, from 12:1 through the end he deals primarily with practical application, i.e. what they are to do in light of the truths he has just expounded.
Although it may not be quite as clearly delineated in his other letters, it can still be seen. One of the most important markers that demonstrates this is his frequent use of the word "therefore" throughout every letter. Usually, this is Paul's way of saying, "now that I have explained this to you, this is how it should be reflected in your daily life."
Christians today often err in this regard in one of two extremes: either by minimizing the importance if theology as if its only of interest to academics, or by treating it merely as something to be studied, information to be known, and it having little or no impact upon how we live our lives. The former is probably the more common error in American Evangelicalism today, whereas the latter is the tendency of those in Fundamentalist circles. While maintaining the proper balance between these two can be tricky, I am convinced it is essential for our walk of faith.