Supernatural spent years leading viewers to believe that the archangel Michael and the Winchesters' half-brother Adam had gone insane, but a dramatic season 15 return proves they're two of the most well-balanced characters in the series. Michael is first mentioned in Supernatural season 4, when Sam and Dean are revealed as the purpose-built vessels for Lucifer and Michael, respectively, to take human form and do battle with each other. Unfortunately, this battle would bring about the apocalypse, so the Winchesters are forced to put a stop to the divine heavyweight clash by casting Lucifer (possessing Sam) and Michael (possessing Adam) into Hell's infamous Cage.
While both Lucifer and Sam managed to escape The Cage before too long, Adam and Michael remained locked in there as of Supernatural's season 5 finale. Since that episode almost a decade ago, fans have had to make do with passing comments and brief allusions with regards to Michael's fate, while the Winchester brothers more or less forgot Adam even existed. In season 6, Castiel suggested that Michael would team up with Lucifer in The Cage and torture Sam Winchester's soul for spoiling their plan. In season 11, Lucifer told the Winchesters that Michael was driven mad by life and became a shell of his former self. Shortly before the battle with the Darkness, God claims Michael is in "no condition to fight." In short, Supernatural repeatedly told fans Michael was a mess.
Both the archangel and Adam make their return in Supernatural's final season, when God opens the doors of Hell, and Sam and Dean are clearly panicked about this possibility. They'd only just managed to kill the Apocalypse World's Michael, the last thing they needed was an insane archangel on the loose. However, when Michael makes his eventual return in "Our God, Who Aren't In Heaven," the archangel and his human vessel are absolutely fine. More than that, Michael and Adam are working in harmony as a near-perfect tandem, with both angel and vessel respecting the other and regularly swapping over control. This is virtually unheard of in Supernatural, when even willing possessions tend to turn sour before long. Granted, Michael and Adam do sit in a diner and talk to themselves but, crucially, they realize how things must look to the general public, definitively proving they aren't insane - just jaded, annoyed and hungry.
The big question is why Supernatural would perform such a dramatic U-turn on Michael's character. Sam and Dean suggest that Lucifer and God might've been lying about Michael's mental state; after all, one's the devil and the other turned out to be an even bigger villain. Lucifer might've lied about Michael going insane just to make his own escape look more impressive, while God could've gone along with the lie just to keep the Winchesters' story more interesting. This explanation mostly works as an in-narrative reasoning, but it doesn't quite sit comfortably, mainly because there was never any implication or hint that any character was lying about Michael until his return was imminent. No seeds were sown in advance that might've tipped the viewer off, as one would usually expect when a character isn't being truthful.
It is possible that Michael's return is actually a retcon made out of necessity. Supernatural had no plans for Michael prior to season 15, and writing him off as insane was a convenient way to explain both why Sam and Dean couldn't recruit him for big battles and why they didn't try harder to bring back Adam. When Michael was confirmed to return in season 15, Supernatural already had enough plot threads to tie up, so it was easier to make Michael sane than to add another villain into the mix.
As a potentially fascinating alternative, it's plausible that neither Lucifer, nor Supernatural's writers, were lying about Michael being insane. Although the archangel seemed stable upon his return, could he and Adam have spent so long in The Cage that they learned to hide their true madness? Might the true Michael emerge later in season 15, full of latent anger and an unhinged violence that causes Sam and Dean yet another headache.
Supernatural season 15 continues January 23rd on The CW.