Listeners can concentrate when we motivate them to do so. But it is important to remember that it is mentally tiring to maintain intense concentration.
In a conversation we find ourselves checking out now and then, or cracking a joke periodically to bring relief from the intensity. In preaching we need to be considerate of the mental energy of our listeners.
When I was growing up and preaching some early sermons (or versions thereof!), my church decided to believe the hype about concentration spans (i.e. it is impossible for contemporary listeners to concentrate beyond 15 minutes). They were conservative enough to want to keep their 30+ minute sermons, so they decided to break up the sermons with a hymn or chorus at the half-way stage. The logic seems clear enough. The idea was flawed. As a listener I could tell it didn’t work. When I preached I could feel the problem! After singing and switching off for several minutes, the preacher had to re-introduce the sermon in order to get listeners onboard again. Don’t try this at home.
However, listeners do need breaks in the intensity now and then. A good illustration can really help (as long as it is somehow moving the message forward rather than merely pressing pause). Humor carefully used can break tension, release some steam as people take the chance to laugh, then re-engage more willingly. Varying pace, pitch and power of the voice are critical, not to mention the strategic use of pause. In reality people can’t concentrate for even 15 minutes at once, it is more like 3-5 minutes – so carefully shape the sermon in appropriate length movements with very deliberate and careful transitions!
Concentration uses energy, even when people are motivated. So as a preacher don’t simply shrink every sermon or chop it up to allow for commercial breaks. Instead strive to stir motivation (interest, need, thirst), design sermons in suitable movements with careful transitions, and present with an engaging enthusiasm that provides appropriate breaks to keep people with you.