A little consideration of the hearers will go a long way to making sermons more understandable. Are many of your hearers from a different culture to you? Then keep your vocabulary as simple as possible and try to avoid idioms. Are there likely to be those with little to no Bible knowledge? Then don’t use obscure illustrations from the Bible with no explanation. Is there a difficult word in the Bible text itself? Take time to explain it in simple words.
Too many sermons are more complicated than they need to be. Like any specialty, the Christian world has its own special vocabulary. Words like holy, justification and glorification do turn up in the Bible, yet they rarely if ever turn up in everyday conversation. On top of this, there are all kinds of theological terms used to describe Biblical concepts that are rich and meaningful if you know what they mean, like Trinity and ordo salutis and transcendence. Yet the majority of people in our churches listening to sermons don’t know or use these words. Anyone whose task it is to explain the word of God to others needs to think carefully about the words they use lest they are misunderstood.
Think about Jesus in his teaching. He used the common language and common illustrations that everyone would understand. Sure, they might not have understood what he meant, but the words were clear. Likewise, when Paul spoke to the Athenians who were educated but not experienced in the Jewish Scriptures, he spoke plainly. He explained God’s sovereignty and aseity without using those words. It is more than possible to explain difficult concepts in simple words.