Clan requirements are the same as cabal requirements. For instance, only lawful characters may join Militia.
As Anithraril said, one's religion in FL is dependent on alignment and ethos. For instance, a worshipper of Malchaeius would simply be unable to join Militia. There are some exceptions, but we see them as roleplay opportunities.
Taking a fantasy world's concept of ethos and alignment, and comparing it to its counterparts in the real world (i.e. morality? ethics?) is completely unreasonable. Forsaken Lands has strictly defined meanings for what is "good" and what is "evil," for what is "lawful" and what is "chaotic." No such agreement over what consists of moral and ethical behaviour exists in the real world. It is this difference that allows alignment and ethos to define one's religion in FL. In other words, characters in this MUD act a certain way because those actions have been defined.
Letting players choose religion before alignment and ethos would not make a difference because, in the end, they will still have to choose those two traits. And their religion would then force them into certain a/e combinations. For example, a follower of Chayesh would have to choose evil.