[Warning: This post contains spoilers for the Walking Dead comics and may contain spoilers for the series.]
The Walking Dead's Season 8 premiere contained three short scenes of a sweaty, pink-eyed Rick Grimes (Andrew Lincoln) kneeling beneath a piece of stained glass and sort of zoning out. They were paired with soft-focused flash-forwards into the future where Rick's hair is thinner and shorter but his beard is big and bushy and he walks with a cane. Alexandria is at peace; so peaceful, in fact, that they're having a party for which they've built a big owl (maybe the residents of Alexandria are all Illuminati) and are listening to "Weird Al" Yankovic. At the end of the last flash-forward, Rick said "my mercy prevailed over my wrath."
The show did not explain at all what these scenes were supposed to signify, but we have a theory: the scenes with Rick and the stained glass take place at the end of the war immediately after he has decided not to kill Negan (Jeffrey Dean Morgan), and the Old Rick scenes are him indulging in fantasy about what life will be now that the fight is over.
On the show, Rick has his mind made up that he's going to kill Negan. He's said it over and over again, and reminds Negan that he's told him twice. It's basically his animating principle at this point. But in the comics, Rick didn't kill Negan at the end of the "All Out War" storyline. In the comics, Rick convinced Negan that they should all pool their resources and work together, and that Negan's totalitarian way of doing things caused a lot of suffering. Negan admitted he was wrong, and then Rick non-lethally slashed his throat and took him prisoner.
The scenes with pink-eyed Rick could be a flash-forward to right after this moment or whatever the show's similar version of this moment will be. The "my mercy prevailed over my wrath" line indicates that he spared Negan, just like in the comics. The line is a paraphrase of an Islamic hadith, or report about the prophet Muhammad. Rick probably picked this quote up from the new character played by Avi Nash -- the man who was begging Carl (Chandler Riggs) for food -- who quoted the Quran in his first appearance this episode.
And the Old Rick scenes in the future are pure fantasy, like the ones Rick had of the happy dinner table while he was being tortured by Negan in the Season 7 premiere. In the comics, after the war there's a time jump of a couple of years. This could be a reference to that, and also a hint that the post-war era will not play out exactly as it did in the comics.
Or it could be Rick in the midst of war praying that mercy is shown to him after suffering heavy losses. We'll see.
The Walking Dead airs Sundays at 9/8c on AMC.