Circle Series 4-in-1

I didn't get past the first book in the series as I found it long and ponderous. The novel goes between two worlds; one that appears to be in the present day and a fantastical world with colored trees and flying bats. I found myself anxiously waiting to get to the present day each time I was left in the fantasy world, where I thought the novel dragged.

I realized quickly that it was supposed to be a Christian allegory, but I felt it had been done many times before, and more interestingly.