Aida I wouldn't say it's a major focus. Rather, it's a driving force for the parent who shapes a control over a family, all in the name of religion. There are several underlying themes in this book yet one seems to stand out the most. (Once you get past Part 1, It becomes a pretty quick read.)
This answer contains spoilers… (view spoiler)[Yes, I think that religion is a strong theme in this novel, Arode. The main conflicts of the novel have to do with religion. The father has broken with his traditionalist father over religion, and the young son (the protagonist's brother), is in turn, rebelling against his father's literal interpretation of Catholicism. The children's aunt (the father's sister) is a more moderating force in their lives. She is a practicing Catholic, but she is much more inclusive of others' beliefs in her interpretation of Christianity. There is also a warm-hearted priest who helps the protagonist overcome the inhibitions her father has instilled in her with his strict, abusive parenting. (hide spoiler)]
Ariel I wouldn't call religion a theme although it figures prominently. Religion is a significant part of how the characters express themselves and their approach to life.