It's a popular theory, but is yet to be confirmed. If you ask Aglaecwif about what the "greatest power" is, she mentions something called Order that's made by combining Authority and Power, and says that it's just one of an entire group of ultimate abilities. Given that Authority and Power are one of the Apostle pairs, people assumed that every set of Apostles has an ultimate ability made by combining their powers. This idea is reinforced by the poems the MC hears when connecting to the minds of the Apostles in the dream world. Each pair of Apostles mentions a common concept. Since Aurthority and Power's common concept is "order", it's assumed that the common concepts are the names of their combined powers. But it's all just conjecture at this point, nothing has been confirmed.
What is The Arbiter, did i miss something?
The Arbiters are a set of beings that are probably the predecessors to the Apostles. We don't have a ton of information. If you ask Aglaecwif about the gods that monsters worship, she'll say that the oldest monster pray for the return of the Arbiters. Later on, Tanos mentions them when talking about his investigations into the origins of monsters, saying that they're an uncommon myth among monsters that mentions six godlike beings that all the others are descended from. Ella can be asked about them if you heard about them from Aglaecwif, to which she responds that she's surprised Aglaecwif mentioned them and that most monsters seem afraid that talking about them will anger the Apostles. And if you enter the Chaos flame at the temple in the latest update, you see a memory of a being that was once part of a group of six, before three of their member were abducted and split into pairs of twin monsters.
The most common interpretation is that the Arbiters were the original gods of the world, but were somehow split apart into the Apostles. Going with that theory, just as the Apostles are named after the power they hold, the Arbiters would have been named after the combined power made from each set of Apostle powers. There's an accompanying theory that the "Eye", the thing that appears to the MC in visions and takes over in some Dead Ends, might be a surviving fragment of the Arbiter of Evolution. It seems to be made of a mix of Body and Memory, very old monsters recognize it, and it talks about evolution.