The Need for Systematic Theology: To Know It and To Teach It . . .
The Need for Systematic Theology
“Systematic Theology” 
Preparation and Delivery of Sermons
John A. Broadus
Edited and Offered
Roger D Duke
Systematic Theology is of unspeakable importance to the preacher, indispensable if he would be in the best sense instructive and exert an abiding influence over his hearers. This enables him to speak with the boldness of assured conviction, giving him a confidence in the great system of inspired truth which no minute criticism can shake. This prepares him to urge on doctrine, or to unfold and apply one text, without the fear of offending against another—a fault unto which many ministers are grieved to remember how often their early sermons fell. This renders it practicable to discuss particular aspects of a doctrine in different sermons, in such a way as be degrees to import a good knowledge of the doctrine as a whole. . ..
Of other reading, regarded as an important source of materials for preaching, there can be only brief mention. Church History does not usually receive from working ministers the attention it deserves. Especially does the history of Doctrines assist one in understanding the truth, and in comprehending those objections and erroneous tendencies which under different forms reproduce themselves in every age. Every religious denomination has certain characteristic or favorite doctrines, which its standard works bring out with clearness and prominence; . . . that apart from the necessary provision for polemical preaching, and from the common stock of Christian Theology, there is to be learned by studying the peculiar opinions of different denominations. . . .
And so as to all our reading. Young men who have enjoyed by limited opportunities of culture and have never looked with eager eyes upon the great world of books, sometimes need to be urged to read more widely; but in the immense majority of cases, very different advice is required. He who would become really a man must abandon as early as possible the childish dream of reading everything. Except what is done for recreation—he must have a limited field of study and must cultivate that field with the utmost possible thoroughness. And upon every subject studied, he must find out the best books, and restrict himself almost entirely to those. If the men of true scholarship and real power were called on to give counsel to young students, in this age of multiplied books, they would probably all unite in saying, Read only the best works of the great authors, and so read these as to make them thoroughly and permanently your own. . . .
[T]here are other sources of materials for preaching, besides books. A preacher’s knowledge of human nature, and knowledge of the world, his experience of life, and especially of the religious life, his conversation with those around him upon religious and upon general themes, his perpetual reflection upon everything felt, observed, or heard—these afford a large part of his most valuable materials. And all his previous preaching, if rightly managed, has but enriched the mind to meet further demands. If one merely scrapes together thoughts around a subject, so as to make a sermon, then every sermon consumes part of his material, and leaves him poorer. But, if he habitually penetrates into a subject and masters it, every sermon leaves him richer; not that he can shortly preach again upon the same topic or text, but that he is better prepared for treating others akin to it. There is a fertilizing production. It this sense too, “there is that scattereth, and yet incresaeth.”
 John A. Broadus, A Treatise on the Preparation and Delivery of Sermons, 2nd ed. (Philadelphia: Smith, English & Co., 1871), 122 ff.