I had picked out the traitor by ten minutes into the first episode. Now I admit by half way through the first season I'd given up on the idea, but by the end of the season I was proven right. Which says more about the leisurely plot development than anything else. And that, my friends, is weak writing if the quisling can be spotted that quickly.
Our heroine Ava (Alba Baptista) is 17 years old, a (American?) tetraplegic in a religious orphanage in Spain. We also see the nuns of the "Order of the Cruciform Sword" in a fight that eventually kills their leader. As the attack against the nuns makes succession difficult, a desperate bid to save the holy relic that made the nun's leader powerful sees the relic end up inside Ava's corpse. This brings her back to life and gives her some interesting superpowers. She doesn't know the source of these powers, but does know she can walk and run, and goes out to enjoy life.
That's the first episode. The series then spends the entire rest of the season on A) showing us what Ava's like (has no faith in herself, likeable but very sarcastic, a not very good actress), B) setting up the idea of "Divinium," an organic material of incredible power, C) setting up the internal political struggle in the church around the Order of the Cruciform Sword, D) setting up the existence of multiple types of demons and how they can be killed (usually only by the Warrior Nun herself) and E) setting up Ava's eventual reluctant joining the Order.
As other critics have said, the fight sequences are better than usual for TV. The problem is everything in between. I don't mind preposterous story lines: I love SF, Fantasy, Superheroes, I watch it all. But it's kind of important to actually hold the storyline together with good dialogue and a plot that makes sense. The plot ... mostly makes sense, but the dialogue is weak, wooden, and delivered by a cast that couldn't sell umbrellas in a downpour. Alba Baptista is pretty and has got the basics of acting, but seems to have missed out on the advanced class.
And with ten hours invested in watching the series, something finally actually happens in the last episode. Much is revealed (although certainly not everything, no, not everything), and we're left with a Grade A cliffhanger. And I find I really don't care what happens.
One of the few things I agreed with in the series was its apparent dislike of organized religion: I'm not a religious person, but I have a lot more trouble with the organized churches than with belief in a deity. But even that looks like it's going down the tubes as the big twist in the final episode throws that into doubt too.