One of the great temptations in preaching a sermon is to find a political point you wish to make and then find a story from the Bible with which you wish to illustrate it. over the past few weeks preachers have been seizing on the story of Jesus overturning the tables of the moneychangers in the Temple to preach a sermon about the unfairness of the inequality in the financial sector - quite out of context. Pick a text out of context and use it as a pretext.
Equally invalid is to preach systematic theology, using illustrative passages. This is practised by Evangelical preachers throughout the world. Proper preachers should be preaching Biblical theology - preaching what actually comes out of the passage.
To do this well it is necessary to properly divide up the word of God. There is a structure that the original authors applied to Scripture and it is necessary to sort this out from the chapters, verses and paragraphs that later editors have imposed.
You can find a superb example of how to divide up Scripture in the preaching of Chris Kelly, particularly in the Gospels of Mark and John. and I thoroughly recommend it.