Commentaries are resources for preachers, not sources for sermons. They are tools that help us in the passage study phase of our preparation. They are not a sermon bank of material waiting to be pilfered and preached.
If you read the introductory preface to a commentary (which would be unusual behaviour, I suspect!), you will see that the commentary or series is targeted toward a specific audience. Perhaps it is aimed at non-Greek trained lay people, or at seminarians, pastors, and Bible teachers with some Greek, or whatever. In reality, these categories are so broad that I would prefer to view them not as targeted communication, but as descriptions of a range within which the writer offers his or her explanation.
Preaching is different. When you preach your goal is not just explanation to a broad audience, but targeted transformation in a specific audience. You can be much more specific in knowing whom your listeners are and what they need to hear — not only by way of explanation, but also with an emphasis on application.
Here are three more related comments on preaching and commentaries:
1. Watch out for atomization.
The vast majority of commentaries are highly atomistic. While a good commentator will be aware of the discourse level unity of the passage, it is hard to find commentaries that are overtly aware of the macro level flow within a book. It seems to me that often the commentator is so engrossed in the phrase-by-phrase explanation, that a stretch and coffee break before proceeding with the writing can lead to a sense of atomization in the end product. The preacher is not offering a book in which the listener can go back and review the section introduction or re-read complex sentences. The preacher is offering an aural exposure to both explanation and application of a text. Different.
2. Only quote a commentary if the quote is exceptionally valuable.
You don’t need to prove that you read commentaries (or checked in with Calvin, or whomever). You don’t need to feel inadequate to be the preacher (though we all are) — they invited you to preach, not Doug Moo or Tom Schreiner. Study and prepare to the point that you can effectively explain and apply the text. Only quote a sentence or two from a commentary if it really is uniquely pithy, arresting, compelling and gripping, not to mention helpful!
3. Don’t feel obligated to cite your sources.
If you do quote, no need to cite sources every time. Preaching is not an academic essay. Sometimes the reference to an unknown name can be unhelpful, sometimes (depending on the name), downright distracting or humourous! If the author makes a difference, cite them (i.e., Churchill), but if not, just say “one writer put it like this …” (anyone who cares can always ask you afterward).