I know some people are disappointed that it was a side character who saved the planet in 15.19. But friends, Sam Winchester saved the world. Not just with the way he tricked both the Archangel Michael and God Himself, which was amazing. But I've said this before, and I'll say it again: if Jack saves the world, it's because of Sam.
Sam is the one who stopped Dean from shooting Jack right after he was born. Sam is the one who sat in the jail cell with Jack, scared shitless, so frightened that he was practically trying to melt into the wall behind him, and apologized for trying to hurt him and asked if he was okay. Sam is the one who defended him from angels in that cell. Dean was ready to kill him from day one (and I can't fault him for that, after all, it's Lucifer's son) and Sam is the one who said no, wait, let's not blame him for his blood and his heritage and his alleged destiny. Let's give him the opportunity to be good. Let's make him one of us. Let's love him. Yes, Cas said all of that too, but Cas had the benefit of the vision he received before Jack was born. All Sam had was his inherent decency and his faith.
And the part that makes my heart hurt is that basically, Sam gave Jack what he needed himself, and didn't get. The unconditional* support and forgiveness and acceptance he needed back when he was the one who abandoned his family, the boy with the demon blood, the suit Lucifer was going to wear to the prom.
(*Yes, Sam agreed to lock Jack in the ma'lak box, but only when he thought Jack was incapable of stopping himself. Not to punish Jack for killing his mother, but to make sure a gun with no safety was locked harmlessly away.)
If Sam hadn't stopped Dean from trying to kill Jack, the nephilim would have been scared and angry and antagonized. He would have been their enemy. Asmodeus would have obtained the perfect weapon. Sam's faith in God and angels may have kicked him in the face, but his faith in Jack paid off.
So make no mistake, friends. Sam Winchester just saved the world. Again.
(I'm so afraid of what they're going to do to him.)
Do you need to see terrified Sam in the cell again? Because I sure do.