What is the premise? Well, essentially, it is all built around the faith of the Forgefather. It's best described, at least in my mind, as a combination of Tolkiens' dwarves with Catholicism. The Forgefather's faithful worship smithing and mining while using it as a metaphor for their religion. This isn't so crazy as it sounds since Christianity does the exact same thing with carpentry and fishing but the Forgefather faith takes it one step further by actually engaging in those trades. Unfortunately, the Forgefather has been silent for many years and his religion has degenerated into what is essentially an enormous scam. Children are sold into slavery by their parents and these individuals are sent down into the mines to labor for their god until they die.
The Forgefather's seat is less the Vatican and more the Lonely Mountain crossed with Moria as the church sits on top of a massive series mines containing many valuable minerals. If you're extremely lucky, you'll get called up to study in the main temple and enjoy the benefits of not being worked to death. If you're not, then you will only enjoy the benefits of brothels and drugs granted to the faithful by the criminals who assist the church. Even getting up to the main temple doesn't work out for our heroes very well because, in what is the author going after low-hanging fruit in my opinion, one of the chief priests is a pedophile.
Scam religions are nothing knew for Graham Austin King to tackle and there's one which I felt was underdeveloped in his Riven Wyrde Saga. I always felt he missed out by not having a perspective of the believers which the fae were manipulating in that book. Here, the issue is slightly difference because while the present-day Forgefather church is hopelessly corrupt, there's a serious question whether the religion used to be something positive and whether the Forgefather may have abandoned his clergy because they're monstrous asshats.
I have some issues with Faithless but none of these have anything to do with its handling of religion. I'm a theist and have studied both mythology as well as religious history so questions of corruption and reform in clerical organizations (especially fantasy ones) are fascinating. I think Graham Austin King does an exceptional job in examining the issues of his fictional religion from multiple perspectives. I would have maybe preferred a perspective from someone genuinely religious in his books (all of his perspectives are from people with a somewhat cynical view of the Forgefather's faith) but this is a small complaint.
Whether you believe in God, gods, the Force, science, or nothing at all--it's a self-evident fact people can use faith as a cover for their own gain. The book isn't an attack on religion and could have actually gone a bit further in people being angry at the Forgefather or the concept of it in my opinion. Surprisingly, for all the evils of the religion, very few people seem to the Forgefather doesn't exist or are particularly angry at his absence.
Really, my issues with Faithless are two fold: 1. Roughly about halfway through the book, the story shifts from dealing with the corruption of the temple and the hellish existence in its mines to dealing with a supernatural terror. It's a bit like watching The Name of the Rose (a 1986 Sean Connery movie adapting the Umberto Eco novel about corruption in the Catholic Church) and then like in From Dusk Til Dawn, vampires attack the monastery. 2. Making the central villain into a pedophile does feel like going for the obvious most recent memory example of clerical abuses. The book is also a bit strange in its story progression and I think a clearer time differentiation would have helped.
The protagonists, Wynne and Kharios, are perfectly serviceable protagonists. I'm not going to lie, neither of them made a particularly deep impression on me. Both of them are more interested in trying to keep their heads down and surviving the mines (as well as escaping them) than any of the larger issues going on around them. There's some good moments with them, though, like Wynne finding himself sold by his father for a handful of silver to become a slave (all the while being told he's going to join the temple). I'm also a big fan of the fact Kharios' biggest problems all come from the fact he's not nearly as much of a cynical bastard as he likes to think he is and his worst problems all stem from trying to do the right thing in a really ill-considered way.
In conclusion, it's a solid novel that I recommend with some caveats like the fact it suffers from a sudden genre change when I wanted to stick with the corruption in the temple versus monster slaying. Oh well, the White Walkers had to attack Westeros someday.