In short... 1 Corinthians 15.

It's pretty obvious that in the letter Paul received from the church at Corinth, someone must have asked about whether there really is a resurrection of the dead. Or asked about someone who was teaching this. Paul's answer and supporting argument is very extended, and in it he almost teases us with certain theological ideas. I'll get to those in a moment.

Paul begins by reminding the Corinthians why they're even communicating with Paul -- the good news. In a litany like way he states that good news: Christ died for our sins, was buried, and is raised on the third day. (Yes, the Greek means "is raised or has been raised.") He appeared to many, including finally, Paul. Since the Corinthians can't deny this good news, Paul then argues for the resurrection of the dead, on the basis that Jesus was/is resurrected. And it's our belief in this good news, that appropriates the gift of Jesus' sacrifice and saves us. All attested to by scriptures and by witnesses.
That seems pretty straightforward, but Paul likes to, "Dog pile on the rabbit." And as with so many of Paul's other arguments, there are the teasers. For example, in verse 29 Paul writes, "Otherwise, what will those do who are baptized for the dead?" Baptized for the dead? Hmmmm. What's that all about? Quick, consult the thesaurus, find the Greek words, use the Google. Well, there is lots of speculation, and most assuredly we can't tell from Paul's reference whether he actually approved of the practice, but in the end, I'm left with this answer -- I don't know.

Here's another. In verse 2, Paul writes, "by which [the gospel] you are being saved, if you hold firmly to the message I preached to you – unless you believed in vain." Two questions arise from that: "Aren't we already saved?"; and "What does Paul mean by 'in vain?'" I'll save writing about the former until we read 1 Peter. As to the latter, there are at least two ways of translating the Greek. One implies that the person never really believed, and the other that the belief was in vain if the gospel isn't true. In either case, you're not being saved.

There are a couple of other teasers inside this chapter, but I'll leave them for you to discover and wrestle with. After all these supporting arguments in favor of the resurrection of the dead, we get to Paul's practical advice, based on the implications in the good news. It's the bottom line, and I want to quote once again from Andy Gaus' translation, because of the phrase he uses conveys that bottom line quality better than the usual translations of "so then" and "therefore." Verse 58: "In short, my dear brothers and sisters, be firm, immovable, contributing generously to the Lord's work always, knowing that your labors don't count for nothing in the Lord."

Thanks for that advice Paul. Sometimes we get too carried away worrying about the non-essentials. In short, Christ died and was raised, so get to work. Got it.