The Unity of Scripture

My pastor wants me to write a sermon so that I'll have it if he can't be there sometime which happens fairly often because he's a cop and sometimes he gets called out.

So, I thought about writing a sermon concerning the unity of scripture. I want the gospel to be the central focus so I'm going to talk about OT prophecies that are fulfilled in Christ and how that OT believers looked forward to the cross and how that we look back to the cross. But, I also want to focus on the misconception that some people have the God of the OT is a different God than the God of the NT. So, I want to talk about how that the OT does bring out God's wrath and holiness more than the NT, but how that in the OT God is still merciful. And how that the NT does tend to emphasize God's mercy more, but we still see God's wrath for instance when Ananias and Sapphira are struck dead by God and then some believers are dying because they're partaking of the Lord's table in an unworthy manner, and especially the emphasis on God's wrath in the book of Revelation. I also plan on talking about how that the Bible begins and ends with a wedding. I think that will be the springboard into talking about creation, fall, redemption, and restoration.

Also, I want to talk about how the OT seems to suggest in a lot of places that although salvation is of the Jews, it was always meant to be taken to the nations. John Piper covers that in his book, "Let The Nations Be Glad!". Also, the story of Jonah is a great foreshadowing of Christ and how that He was accepted more by the gentiles than He was by His own people.

Anyway, I don't want to put too much in there because I want the sermon to be unified and I don't want it to be cluttered by stuff that would be better used in another sermon.

Does anyone have any advice? It would be greatly appreciated.

---------- Post added at 07:05 AM ---------- Previous post was at 06:56 AM ----------

BTW: I've never preached a topical sermon like this. I'm not quite sure where to start. For one thing, do I still need a main text even though it's a very broad theme?

Even though it's a broad theme, you should still have a main text. A sermon should always be preached from a text, even if you will borrow from several other verses. You may want to use one of the instances where Jesus talks about what was spoken of him in the books of Moses, Prophets, and the Psalms.

Using a specific text helps guard against cherrypicking verses to fit a theme which may be going in the wrong direction. Your text will help to focus your sermon, and still allow you to preach on what interests you.