Short synopsis: Josh Wheaton (played by Shane Harper) is a college freshman who finds himself enrolled in Professor Radisson's (Kevin Sorbo) philosophy class. Radisson is a devout atheist, and on the first day of class requires every student to write "God is dead" on a sheet of paper and sign it. Josh finds he cannot do so; it conflicts with his Christian beliefs. Radisson then offers a philosophical challenge: If Josh will not admit that God is dead, then he must debate and prove the "unprovable" opposite thesis — that God is not dead.
The movie follows several individuals and groups directly or tangentially connected to Josh and/or Radisson, and their journeys toward Christ and God as this challenge plays out. Also featured are Willie and Korie Robertson (of Duck Dynasty fame) and the Newsboys.
The characters (other than the Robertsons and the Newsboys, who appear as themselves) are believable and endearing. You empathize with them (at least I did); there wasn't a single named character you don't feel for, one way or another. Even the professor, who's kind of an ass (OK, not "kind of"), with his "Tony Stark" style and "Tony Stark +10" arrogance (seriously); you want to just hate the guy, but you can't help but feel sorry for him, even before you learn his back-story. And while the story and dialogue have a very strong pro-Christianity bias, I wouldn't say it's excessive to the point of being a propaganda film, which is how many critics described it*.
I'd give it a solid 4.5 out of 5. Great movie, highly recommended.
* - Full disclosure: I'm a Christian, so I can't be fully trusted to remain 100% objective on this point. That said, some clearly pro-Christian films have and do put me off by being over-the-top propaganda. This is not one of them.