Basically what happened at the end of Forever Evil is that the Justice Leagues failed to save the world from the Crime Syndicate and the Secret Society so it was up to Batman and Lex Luthor's Injustice League to save the world. As a result the Justice League of America disbanded, the Justice League's popularity is way down while Lex Luthor's popularity is sky high.
But Luthor also matured quite a bit, on top of feeding his ego by joining the Justice League, he recognizes that Superman is not the biggest threat to Earth but whatever destroyed Earth-3 is (the Anti-Monitor) as a result he sees teaming up with the Justice League as a necessary thing to tackle the impending threat. He gives the Justice League a new Watchtower and Shazam as a member to build up some trust but due to their natural distrust of Luthor, they refuse to give him membership.
So it appears that what will happen is that Luthor will use his knowledge of Batman's secret identity to blackmail his way onto the team and Superman will quit in protest.
