Is the novel Life of Pi a religious book?
Charlotte Innes noted that Life of Pi is "a religious book that makes sense to a nonreligious person." I don't see Martel's story as a religious book. I know that Pi, as a young person, immerses himself into three religions, but once he leaves for Canada, religion is no longer the focus of Pi's life.
Life of Pi may surely be seen as a religious book--and perhaps one for nonreligious people but certainly also one for religious people--for several reasons. The first reason is that the author reports it as a religious book. The reason this must be taken seriously when asking whether it is a religious book or not is that you are asking about intent, and, despite contemporary literary criticisms, when an a contemporary, living, breathing author suggests that his book is by intent religious in nature, it is wisest to take him at his word:
I received a tape and a report from the Japanese Ministry of Transport. It was as I listened to that tape that I agreed with Mr. Adirubasamy [the original teller of the tale] that this was, indeed, a story to make you believe in God. (Yann Martel, Introduction to Life of Pi)
The second reason is that it is a tale told about and through the voice and eyes of a religious man who experiences life in religious ways. Not only did Pi learn and practice three branches of religion he continues (up to the time of the story being printed) to practice his triple religion. If Pi attests to the power of religion in his life and this story, then, despite influences of contemporary literary criticism, it is wisest to believe that the book is a religious one:
My suffering left me sad and gloomy.
Academic study and the steady, mindful practice of religion slowly brought me back to life. I have kept up what some people would consider my strange religious practices. ... and took a double-major Bachelor's degree ... [in] religious studies and zoology. (Chapter 1)
The third reason is that through thematic issues, the book self-reports itself as religious and as one that asks a person to believe in God. In that case, if the thematic structure aims at discussion of religion and a belief in God, then, in accord with New Criticism and its foundations in Formalism and in accord with linguistic literary analysis, it is fairly certain that it is correct to say this is a religious book.
the three-toed sloth ... [that] miracle of life, reminded me of God. ... Life is so beautiful that death has fallen in love with it, a jealous, possessive love that grabs at what it can. But Life leaps over oblivion [of death] lightly, losing only a thing or two of no importance, ... To choose doubt [in God as Kumar] as a philosophy of life is akin to choosing immobility as a means of transportation.
Pi or π is one of the most important mathematical constants, approximately equal to 3.14159. It represents the ratio of any circle's circumference to its diameter in Euclidean geometry, which is the same as the ratio of a circle's area to the square of its radius. Many formulas from mathematics, science, and engineering involve π. So having a character named Pi suggests a religious theme with his life gone full circle and he ends up back where he started but with fullness of life behind him.