Even though Paul looks down on Christians bragging about their accomplishments, he does say that there are a few instances where it's good to get a little boastful.
Basically, Christians can brag about what God has done for them and their hope that one day they'll be hanging with him in Heaven. Note: the word is "hope" not "certainty."
They can also brag about their suffering because, hey, what doesn't kill you only makes you stronger, right? Plus, who doesn't want to hear a good I-was-almost-stoned-to-death-for-my-faith story?
Oh, and of course, Christians should talk up what God has done for them by sending Jesus. This is a big one. Christians should actually probably be talking about this a lot. Paul certainly does. See, Paul says, back before Jesus, people were terrible and sinful, but God loved us so much that he wanted to help us.
Just think, if God loved the whole world back when we were just huge jerks to him, just think how much he digs us now that we're walking the straight and narrow. He definitely hearts us now. A lot.
Okay, but how can just one person's actions (i.e., Jesus' death) change the whole world? Well, Paul tells us, just look at Adam. Okay, we're listening…
See, silly Adam just couldn't resist taking a little nibble of that fruit (even though that was literally the one thing God asked him not to do). And because Adam couldn't follow a simple direction, he and Eve and everyone else that came after them got served one big eviction notice for Paradise. Oh, and they got to die, too. Nice work.
After that, everyone sinned because of Adam's sin. In fact, sin grew and grew until the situation was about a million times worse than it had ever been. Even having Jewish law around to follow didn't make it any better. It was a sin-plosion.
But then God sent Jesus into the world. And Jesus gave up his life to wipe away all the sins that came before him. All in a day's work.
So one guy brought sin and death into the world. Thousands of years later, another guy brought salvation and eternal life to the world. Well, we know which one we're addressing the thank you card to.