What is the objective of religion?
I believe that the original objective (stone age cultures) was probably as a way to understand nature and to interact with it and possibly control it. Essentially it was the science of the stone age. Early people observed nature and came up with theories for why things happened, then tried to affect their world the same way they interacted with each other, by pleading and bargaining. Unfortunately as time went by the original objective became corrupted by the secondary affect of manipulating others by "speaking for the gods" (priests and stories), and thus the institutionalization of religion came to be.
Why would a god require his followers to congregate together?
Gods took many forms, this was by no means a generic requirement, or even a common one. Most of the early theologies had multiple gods, not just one.
What should be the basis of a religion?
The basis of a religious institution should be to strengthen and propagate itself. The same for any institution which hopes to survive. This may not be a "good" thing, but it's the way it works out.
The basis of a personal religious experience should be to strengthen and facilitate the happiness and survival of the individual (in my opinion).
What identifying marks would a religion need to have?
I'm not certain, but I believe all religions need to have at their heart a deity or deities which are ultimately supernatural. But I'm not certain about that, I guess it would depend on which definition of religion you are working with. But the definition you choose would determine the answer to this one.