It is a principle of effective communication.
Check out the Economist magazine. It is right there - always there - between the title of the article and the body of the article. A simple, single, summarising sentence. One of the secrets for TED-talk effectiveness is that a given talk can be captured in the length of a tweet. 140 characters. The doctoral thesis needs this sentence - and so does the sermon.
Haddon Robinson calls it the 'big idea'. Alec Motyer asks, 'What is the one thing that all the other things are about?' Gary Millar and Phil Campbell speak of the 'freshly squeezed essence of the passage'. Every sermon needs it. The thesis. The proposition. To articulate it is a burden, but as the time of delivery approaches it becomes a saviour.
But it is not enough. I am increasingly persuaded that it is not enough.
The best biblical preaching is science - and art. It is prose - and poetry. It engages the mind, but also the imagination. What is said needs to be seen as well. Sadly, those most committed to biblical, or expository, preaching are often those with the least active imaginations. This should not be so. It is not good enough.
This is why in recent years I have required students to include a prevailing image in their sermons. A picture over which they linger. Maybe it is there in the introduction - and then again in the conclusion. Maybe they sink into this image in the middle of the sermon somewhere. Maybe it is tied closely to the application. However it happens, people walk away from the sermon with their imaginations alive.
The best example I know of this happening is in the Bible itself. In the Psalms of Ascent (Psalms 120-134, what I like to call Hillsongs: the original soundtrack), we find prevailing images. Again and again, in these brief little psalms, the songwriter engages the imagination with pictures...
Here is my Top Ten, the prevailing images in these psalms which engage my imagination the most:
Psalm 129: 'those who plough have ploughed my back and made their furrows long' (3).
Psalm 124: 'the flood engulfed us ... the torrent ... the raging waters swept us away' (4).
Psalm 131: 'Like a baby content in its mother's arms, my soul is a baby content' (2, The Message).
Psalm 125: 'As the mountains surround Jerusalem, so the LORD surrounds his people' (2).
Psalm 126: 'And now, God, do it again - bring rain to our drought-stricken lives' (4, The Message).
Psalm 133: 'It is like precious oil poured on the head, running down on the beard' (2).
Psalm 129: 'May they be like grass on the roof, which withers before it can grow' (6).
Psalm 120: 'He will punish you with a warrior's sharp arrows, with burning coals of the broom tree' (4).
Psalm 130: ''I wait for you more eagerly than a soldier on guard duty waits for the dawn' (6, CEV).
Psalm 124: 'We have escaped like a bird out of the fowler's snare' (7).
Here is a Top Ten without even mentioning 'your wife is like a fruitful vine within your house; your children like olive shoots round your table' (128.3). Oops, we now have a First Eleven.
If these psalms, with just a handful of verses in them, can find space for a prevailing image, so can our sermons. Notice, too, how utterly ordinary and everyday these images are. The advantage of being ordinary and everyday is that they have a better chance of engaging the imaginations of a wider range of people. Everybody relates to the ordinary and everyday ... that is what those words mean!
Arguably, the most frequently asked question in the training of preachers is 'where do I find illustrations?' Well, start by investing in your own imagination by seeing the spiritually significant in the utterly ordinary and everyday.