There has been some discussion of this topid here:
Joining an organization and not being religious and here:
Christianity in ritual.
You'll see in that second thread some discussion of the fact that Kappa Sigma (perhaps reflecting some Masonic influence) requires belief in God. The specifics of that belief are up to the individual. There may be other GLOs that, again reflecting some Masonic or other influence, require belief in a Supreme Being. My Fraternity does not.