The Devas are somewhere between a nation and a religion, and one of the two great perpetrators of the Dimension War.
Deva propaganda holds that the many branes of reality are broken shards of the perfect universe that existed before this one. An unthinkably powerful extrauniversal creature now known only as the Corruptor saw the utopia, and desired to conquer it. The monster was eventually banished from reality, but the remnants of its physical form rapidly corrupted space and time, rewriting every law of the universe to make it impossible to not cause suffering with every action. Eventually the universe could no longer hold itself together and completely broke down.
The leader of the Devas is stated by their propaganda to be one of the very few beings who was powerful enough to survive the destruction of reality, indeed, they're believed to have also been nearly as old as that universe itself. They supposedly can alter the very fabric of reality above themselves (leading some to believe that this being is actually an extremely powerful Infokinetic, although this is suspected for every single supernatural figure that hasn't been confirmed to be merely magical), and possess a sufficient understanding of reality's inner workings that they may be able to fix the universe once the Devas are powerful enough.
Devaist critics of mainstream Devaism state that their "divine leader" may have been just as corrupted as anything else, and that even postutopian beings may be able to repair the universe without such unreliable assistance.
The central pillar of Deva ideology holds that any action can be justified if it can be shown to eventually result in a greater amount of happiness than if it weren't taken. This is what justifies their desire to bring the entire universe under Deva control, bringing them to commit acts of invasion, slavery, cannibalism, brainwashing and outright genocide because the most simple mathematics show that the infinite amount of happiness created from them ruling the universe outweighs the temporary suffering that their actions bring. Indeed, the only sin in their eyes is disobedience to one's superiors. Some like Ameratsu feel that the mainstream Deva leaders are only using this as a justification for megalomania, and choose to rebel from them while still considering themselves to be Devas.
Deva writing is heavily pictographic. This means that reading on more complex subjects tend to require usage of large dictionaries, this has the simple effect of blocking unnecessary information from the knowledge of citizens and keeping them under control.