Mists of Avalon is one of those books everyone at all interested in genre fiction probably should read at some point. It's kind of a classic. Not only does it take on the King Arthur mythos, but it does it from the perspective of the women in the story rather than from the men's perspectives. That's why it's also considered a feminist work--it's telling the female side of a traditionally very male story about chivalry and conquest.
It's also one of the few stories out there that features a female anti-hero. I read a few reviews on Goodreads that seemed upset that Morgain (as her name is spelled in by Bradley) isn't the hero of the story. In fact, in the end she's wrong and because she persists in her wrong-headedness she destroys the lives of people she cares about and contributes to the destruction of Avalon.
Both Merlins (Taliesin and Kevin) are of the opinion some sort of accommodation should be found with Christians, that Christianity itself is not the problem--all Gods are one God, after all--but rather the strict interpretation and application of dogma that is a problem. The actions of both Merlins indicates they're trying to undermine the strictness of their Christian counterparts through gentle discussion. Viviane, and Morgaine after her, don't share the Merlins' optimism that Christians can be convinced to moderate their stance against Paganism. (Or, perhaps, don't understand what the Merlins' goals.) As such, the women take on a more aggressive, more combative approach.
Morgaine and Viviane's combativeness is, I think, very understandable given how anti-woman Christian rhetoric is. The Christianity portrayed in the book (which I don't think is that out of line with history) is anti-sex, anti-pagan, and very rigid in what it deemed appropriate for women. Since these Christian men were pretty much pushing women out of any position of power or authority, or even influence, in the name of their God, I can totally understand why Viviane and Morgaine--who were powerful, influential women under the reign of Avalon--didn't want to engage. They recognized they wouldn't have any pull with Christian Bishops, not just because of their religion, but also because of their gender.
Unfortunately, the resultant anger also left them blind to what the Merlins were doing (particularly Kevin) in preserving Pagan worship through combining the two traditions. Up until the very end Morgaine is unable to see her insistence that Pagan traditions need to be kept pure is the exact mirror of Christian's insistence Paganism must be rooted out. Finally, in that last chapter she recognizes that an altered form of Goddess worship has been preserved.
Morgain wonders why the Goddess allowed Avalon to fall. As a reader (and a modern person privy to history) it's clear Avalon is going to fall no matter what. It's also clear that Morgain's actions (particularly her stubbornness and desire for purity) contribute to that fall.
It's kinda tragic.
As a feminist I think it's great Bradley gives such a tragic role to Morgaine. She's our protagonist and we love her for championing women, but we also get to see that her insistence that things should be preserved the way she wants them isn't effective. Not that accommodation is always the answer--sometimes you do need to fight--but in the long term it is the gentle word, the subtle and seemingly innocuous idea that changes the world.
Anyway, that's not exactly where I wanted to go with this. I loved Morgaine and Viviane, heartless and stubborn as they were. Morgause is fantastically evil and despicable.
Gwenhwyfar is more challenging. I think we're supposed to at least feel sorry for her with the infertility and whatnot, but mostly she comes off as irrational and fundamentalist. It's too bad Bradley never gives us a scene of Gwenhwyfar experiencing something positive from her religion. Instead, all we see is her wholeheartedly embracing a religion that does nothing but tell her what a terrible human being she is for being female, for being infertile, and for loving a man other than her husband. Again, tragic, but I think more in that Christianity in general is given short shrift by Bradley's refusal to give someone a positive experience with the religion. Really, people aren't dumb enough to join a religion that gives them nothing.