R.L. Dabney (1820-1898) was an American theologian whose book on Sacred Rhetoric (reprinted in 1999 under the title Evangelical Eloquence) is a fine guide to preaching.  According to Dabney, the cardinal pre-requisites of the sermon are:-

1.  Textual Fidelity.  ‘The preacher is a herald.  The first quality of the good herald is the faithful delivery of the very mind of his king.’  Textual fidelity not only ensures faithfulness to the mind of God, but also gives fruitfulness and variety of matter. …