A: No...and yes. Let me explain. I'll try my best to be vague enough to not reveal spoilers.
If you think that A Thousand Splendid Suns is a religious book-a Muslim book-then The Soulkeepers is probably a religious book.
If you think The DaVinci Code is a religious book, then yes The Soulkeepers is probably a religious book.
You could make some comparisons between The Shack and The Soulkeepers but the later is definitely not as religious or as prescriptive.
In a recent blog post, Elizabeth Ann West mentioned the cross in my picture. I am Christian(Lutheran to be exact) as are most of the people where I live in Central Illinois, but I never intended for The Soulkeepers to be a prescriptively "Christian" book. I wrote it because I read tons of YA books but in almost all of them, the main character has "never thought much about religion". But statistically in America, around 90% of kids are being raised in households that practice a religion. I wanted my character to reflect that young adults do think about God and an afterlife. However, I also wanted to reflect that this thinking is individual, unique, and not necessarily the same as the beliefs of their family unit.
The Soulkeepers takes place in Paris, Illinois, a tiny town on the border of Indiana. At the beginning of the book, my main character is an atheist, transplanted there from his birthplace in Hawaii. As such, the story takes place in a Christian context, as that is the primary religion in the area. This is similar to A Thousand Splendid Suns taking place in a Muslim context.
The story behind The Soulkeepers is, also, based on a fictionalized account of Judeo-Christian biblical history. I would compare this to The DaVinci Code as it uses rarely heard of religious texts to add to the world building in a fictional setting.
And yes, like the Shack, my main character is not an atheist at the end of my story. However, he is also not part of any one religion. He has a uniquely individual experience that changes him, but that experience isn't prescriptive.
The interesting thing to me has been hearing reader's interpretations. Everyone who has contacted me has interpreted The Soulkeepers in a different way. If you have read it or do read it, I'm interested in your opinion. Do you think of it as a religious book? A Christian book? A spiritual book? Or simply a work of fiction in the context of the American Bible belt?