Generally speaking I urge preachers to stay in their preaching text as they prepare, and as they preach. It is too easy to drift into another passage (or ten) and dissipate the impact of the passage we said we would preach. However one of the exceptions that I do tend to mention is when the passage you are preaching quotes or alludes to or relies in some way on another Bible passage. What then?
Actually, the more we know our Bibles, the more we see by way of allusion as we look at the text. I did an exercise with a group of pastors where we worked through Ephesians 2 and thought about Old Testament passages that might have been in Paul’s thinking as he wrote, or even specific wording that he used. We were coming up with Old Testament passages for almost every verse in the chapter! What to do?
1. In preparation, go to OT passages that may be helpful, but don’t lose your focus on your preaching text. It can be a rich exercise to go back and see the text and context of the fall in Genesis 3, the possible wording from Genesis 6, the session of Christ in Psalm 110, the far and near reference in Isaiah 57, the background of circumcision language in Genesis 17 and elsewhere, etc. But remember that you need to be able to preach Ephesians 2! I may feel like a sawn off shotgun has scattered marks all over the canon, but that is my blessing, not my listener’s burden!
2. In preaching, only go to one or two OT passages if they are genuinely helpful, but don’t lose your focus on your preaching text. Listeners simply cannot handle masses of other references. It turns a sharp and pointed message into an annoying multi-point prodding. If one, or maybe two, are particularly helpful, then use them carefully. In Ephesians 2:1-10, for instance, I’d be inclined to go to Genesis 3 in the early verses, but I wouldn’t chase multiple other references. Perhaps Psalm 110:1 in reference to being seated with Christ. Probably no more. Better to hit home specifically than to scatter shot everywhere.
My personal goal includes getting to know the Word of God as much as possible (not as an end in itself, but since through the Word I can know God). My goal in preaching is not to show that off, but to help people be impacted by this particular text.