Allegory is a great tool for illustrating a truth. Allegory is not evidence for that truth.
Another way to say it is that allegory is designed to bring truth home. It is not designed to defend truth.
To merely preach allegory is to preach without evidence and defence for the truth you are preaching.
An article published in the Biblical Builder recently (vol. 43, issue 3, pp. 18-19) illustrates the point.
The author uses the story of Paul sending Onesimus back to where he belonged as an allegory of the local church and its members. The allegory was good and the author drew many helpful and valid parallels (and I believe some invalid parallels).
But the allegory was just that. An allegory. It had no authority. It was merely a teaching tool. It would have been just as authoritative to use an allegory from a Disney movie.
The authority is in the Word
As Fundamentalist Bible-believers, we understand that the authority is always in the Word of God itself. Using a passage as an allegory of an unrelated truth is easily misleading because it implies authority for the truth we are addressing when in fact, there is none. That is not what the passage is saying.
For instance, the story of Onesimus is not about the believer’s relationship to the church. Not in any way. You can exegete it until you’re blue in the face and that’s not the point of the letter. Not even close.
That’s why I’d prefer the preacher to take his allegory from Disney.
The truth is assumed
The problem with using allegory in teaching a truth is that the truth must either be assumed, or taught in an abbreviated form.
For instance, in the article on Onesimus, the underlying truths are stated and a few proof texts are given, but there is almost no actual teaching from the text of the verses that actually deal with that truth.
Such treatment of the Word, on a systematic basis, will lead to anaemic Christianity because it teaches people what the preacher believes, but provides little or no authority from the text of the Word.
In other words, it’s just a guy giving his opinions.
Those who take God’s Word seriously must evaluate their preaching methods. Do I use God’s Word to preach my message? Or do I preach God’s Word and its message?
There is a place for using allegories in preaching. But we must clarify either that we are assuming a truth we’ve recently carefully taught from another passage or we must take the time to carefully teach the truth in the message itself.
That said, since allegory is not primarily a teaching or admonishing tool (Colossians 1:28), I believe its place in preaching is limited.