Topical sermons and topical series are not the same thing. A topical sermon typically uses multiple texts to make the overall point intended by the preacher. This is not wrong and can be very effective. A topical series could use a single text each week, but not sequentially through a book. This is not wrong and can be very effective.
I am much more wary of topical sermons, however. Why? Because it is much easier to abuse texts when you need to get through three or more of them in the same sermon. You don’t have the time to explain context, develop content, linger over the passage and its impact, etc. The danger is that the text becomes a proof text and a servant to the preacher.
It is possible to preach an expository-topical sermon. This takes significant time to work with each passage and let it be the boss of that section of the message.
It is easier to preach an expository-topical series. It still takes more time than preaching through a book, but working with a focus text each week will allow you to let the text be the boss more easily than when you have multiple texts.
Some readers are in churches where expository-topical series would be a dream (because as things stand every message is non-expository). Other readers are in churches where it takes the annual Nativity season to break the pattern of preaching through books. Here are a couple of nudges for both:
Preach Through A Book: This is how God gave us the Bible – in books. People in our churches need to be people of the books in order to be people of The Book. Working through books means that we will not bypass the awkward or challenging sections. Preachers (and listeners) will benefit from time in a single book as they will hopefully get to know it as a whole and the impact can be reinforced.
Preach Expository-Topical Series (where each message is an exposition of a key text): People in our churches need to grasp the key texts and see how the Bible addresses key issues, or how the values of the church are biblically rooted. Working some series this way means that we will help people see what the Bible is saying without having to remember multiple threads over multiple months. Preachers (and listeners) will benefit from multiple genres and key texts from multiple books that will hopefully motivate them to pursue more for themselves in those places. It may be that a good single message from Jeremiah will motivate people to get into the book, whereas a long series in Jeremiah may put people off returning to it for a while (obviously this works both ways and depends on both the preacher and the listeners!)
I think preaching through books or sections of books is the best staple for a church diet. But I am not convinced we should avoid expository-topical series – judicious use of this approach can be highly effective if used appropriately. Whatever we do, let’s avoid non-expository preaching where the text is not the boss of what is said. That is a move we can’t afford to make.