Charles H. Spurgeon in his book entitled Lectures to my Students calls illustrations Windows of the sermon. Just as a building needs windows to let light into it, so some sermons need illustrations to illuminate and explain what the preacher is trying to communicate to his listeners.
Illustrations are used throughout the Bible, especially when we read the prophets, apostles, disciples, and the Lord Jesus Christ’s preaching. The preachers found in Scripture often used every day events (parables) to enlighten the minds of those that heard them. It is said that people will more likely remember a message if illustrations have been made use of. Such stories and analogies enliven a sermon. They may be humorous, yet not crude, or joking for joking sake. Martyn Lloyd Jones advises that the preacher should not use more than two or three illustrations during his sermon, or else it will quickly become another form of entertainment. Neither should they be used as light-hearted breaks (like commercials) in a sermon. It would be better to leave illustrations out than to misuse them.
It is not a requirement for illustrations and stories to be used during a message. There are some preachers who can effectively teach without them. Yet, nevertheless, we cannot go wrong by following the example laid before us in the word of God where illustration is made abundant use of. There are some sermons were illustrations would be totally inappropriate and could ruin an otherwise good sermon. There is no hard and fast rule to observe in this area, but must be left up to the preacher’s discretion and experience.
In preaching a sermon to a possible mixed congregation (saved and unsaved) I believe that illustrations will help the unsaved understand what we are teaching. Jesus used parables in precisely this fashion … “All these things spake Jesus unto the multitude in parables; and without a parable spake he not unto them” (Matthew 13:34). When plain teaching is being given during a Bible Study, when usually only church members are present, it is not necessary to illustrate points of doctrine … “Unto you it is given to know the mysteries of the kingdom of God: but to others in parables” (Luke 8:10). This of course depends on who the Bible study is geared for. Therefore, illustrations are useful if used correctly, clearly, cleanly, and are relevant to the sermon given.