I don't know any letter that is more fundamental and foundational than Paul's letter to the Romans. It is unquestionably the greatest of all of Paul's letters and the widest in its scope. It is most intent and penetrating in its insight into the understanding of truth; therefore, it is one of the books of the New Testament that every Christian ought to be thoroughly familiar with. If you haven't mastered the book of Romans and aren't able to think through this book without a Bible before you, then I urge you to set that as your goal.