Symbolism, Imagery, Allegory
The cross has become one of the most enduring and well-recognized Christian symbols in the world. You'd almost never guess that it started out as a way to kill people.
So how did the cross get such a great PR makeover? We can start by thanking Paul.
Sign of the Cross
Imagine that you have a teacher you really dig. He's this charismatic guy who's always got great insights about the world. Now imagine that, one day, that teacher is arrested, tried, and convicted of a crime. He's sentenced to death by lethal injection. What would you do? Would you forget about him and just go on with your life? Or would you start walking around with a little syringe around your neck and bragging about how hardcore this guy was for being put to death?
Paul's in the latter camp. Sure, a savior who's died a very embarrassing and humiliating death may not be best selling point for your new religion. But Paul doesn't shy away from it. In fact, he doubles down:
May I never boast of anything except the cross of our Lord Jesus Christ, by which the world has been crucified to me, and I to the world. (Galatians 6:14)
Boast? Yup. One of Paul's favorite things is taking an idea and turning it on its head—and the cross is no different. For Paul, true power is found in humility. And what's more humbling than being executed?
[Jesus] did not regard equality with God as something to be exploited, but emptied himself, taking the form of a slave, being born in human likeness. And being found in human form, he humbled himself and became obedient to the point of death—even death on a cross. (Philippians 2:6-8)
It's in obedience and humility that we draw closer to God, says Paul. The cross represents this. That's why Paul believes that all Christians are called to share in the suffering of Jesus:
• "I have been crucified with Christ." (Galatians 2:19)
• "It was before your eyes that Jesus Christ was publicly exhibited as crucified!" (Galatians 3:1)
• "It is those who want to make a good showing in the flesh that try to compel you to be circumcised—only that they may not be persecuted for the cross of Christ." (Galatians 6:12)
• "Many live as enemies of the cross of Christ; I have often told you of them, and now I tell you even with tears." (Philippians 3:18)
Basically Paul is saying: "So you're being persecuted for following Jesus? No worries! People didn't like what Jesus had to say about God either. That's all part of the deal. Besides, you can bet that Jesus suffered tons on the cross, so all your suffering is just helping you become more and more like him. See, there is a silver lining."
Don't Cross the Romans
For a group of people who would have actually known what death on a cross would have looked like, this is pretty powerful stuff.
Crucifixion was a particularity terrible and humiliating punishment used throughout the Roman Empire. A victim would be stripped naked, have nails pounded through his arms or feet (or both), be placed upright on the cross, and be left to die. Since the nails alone wouldn't kill you, the death was slow. Victims often died from starvation, suffocation, or shock, and depending on the method that was used, death could take hours or days. Not exactly something you want to imagine your Savior going through.
But Paul's strategy obviously worked. It would take a while before Christians would start drawing, painting, or wearing the symbol of the cross, but once they did there was no stopping it. In 321 CE, the Roman Emperor Constantine saw a vision of a cross with the words, "By this, win" floating in the sky. He slapped crosses on all his soldiers' shields, won the battle, and eventually converted to Christianity and took the entire Roman Empire with him (source).
That's a lot of work for one little cross.