Warning: SPOILERS ahead for Justice League!
Justice League is fittingly the restoration of hope in the DC Extended Universe. In the final act of director Zack Snyder's DCEU trilogy, Superman is reborn, given new life thanks to his fellow heroes Batman (Ben Affleck), Wonder Woman (Gal Gadot), The Flash (Ezra Miller), Aquaman (Jason Momoa) and Cyborg (Ray Fisher), who also find themselves at their lowest ebb when even their combined powers aren't enough to defeat Steppenwolf (Ciarán Hinds) and his invading legions from the planet Apokolips.
Superman joins the League in their final battle and turns the tide to help them defeat Steppenwolf and destroy the Unity of the Mother Boxes. Not just that, he makes his presence known by telling Steppenwolf "I like Truth. I also believe in Justice." In that moment, Superman brought with him not just power, but the all-important thing he literally wears on his chest: hope.
One of the most pivotal - and heartbreaking - lines spoken in Batman v Superman: Dawn of Justice was after the Man of Steel failed to prevent the explosion of the U.S. Capitol. This was his lowest point in the film, at least emotionally. Before taking a sabbatical north, where he ultimately had his inner compass set right in a dream sequence where he met his late adoptive father Jonathan Kent (Kevin Costner), Superman (Henry Cavill) visited Lois Lane (Amy Adams) outside her Washington D.C. hotel room. Mournfully, Superman told her "Superman was never real. He's just a dream of a farmer from Kansas." In this moment, Superman had lost his sense of hope.
The DCEU Was Meant To Be Dark
Before Justice League, hope was in desperately short supply in the DCEU. This was by design. As one of the DCEU's primary architects, Zack Snyder considered Batman v Superman the second act of his uber-story about the Man of Steel and his place in the world, as Batman v Superman's assistant director Damon Caro attested:
"The arc was BvS was the midway point and the darker movie... The great stories, the Greek tragedies, even life we have to be knocked down before we can build our way up. Justice League was always the rebirth of hope and the rise. People try to say that it’s a response to the backlash, and I’d definitely say that criticisms were heard, but it’s not like we threw everything out and started with a blank slate."
Fear was built into the DNA of the DCEU from the very beginning, specifically so that it could eventually be overcome. In Man of Steel, even before the Kryptonian invasion, Clark Kent hid from the world and performed his good deeds anonymously, fearful that the world wouldn't accept him if he went public. Despite his saving the world and choosing humanity over his own people, Batman v Superman established that while many saw Superman as a hero and a savior, the most powerful man in the world was a symbol of controversy. His good intentions were constantly questioned and his power was especially feared by political pundits, the military, the government, and especially by the Batman. It was only after Superman sacrificed his life against Doomsday that he began to be honored across the globe the way he deserved.
The Suicide Squad was founded by Amanda Waller (Viola Davis) out of fear that the next Superman wouldn't be as benevolent as Clark Kent. In recruiting dangerous super criminals to serve the government's needs, Waller's Squad was her cynical attempt to wring some kind of hope (that she could control and manipulate) for a country and planet she considered otherwise defenseless from threats of growing power.
It's no coincidence that the minions of Steppenwolf, the Parademons, are creatures who feed on fear. Once Superman was dead, Batman discovered his Knightmare was coming true and the Parademons were infesting Earth. They had a plethora of fear to draw them to Earth, which was experiencing escalating tensions and a rise of criminal and terrorist activity in the wake of Superman's death. Batman and Wonder Woman drew the Justice League together to fight Steppenwolf's invasion, but Batman also inherently understood that what the world needed was the one person who embodied hope more than anyone else - Superman.