1. I don’t think TFiOS has a necessarily secular worldview. It really depends on your reading of the book. Hazel’s dad, for instances, makes the argument that the universe is invested in consciousness, which is not a strictly atheistic thing to say and is in fact perilously close to claiming the existence of heaven.
1a. Of course Hazel dismisses her dad’s argument, so there’s that.
1b. Then again, many of the central events of the novel take place in the Literal Heart of Jesus. Setting a novel inside the heart of God’s son does not strike me as a particularly unChristian thing to do.
1c. Of course the kids are always making fun of the place and claiming that Patrick’s use of the phrase Literal Heart of Jesus is a misuse of literality.
1d. But then again, Hazel and Gus and Isaac themselves come to call the place the Literal Heart.
1e. It seems to me that different characters in the book find varying degrees of secular, religious, theistic, and atheistic ways to confront the reality and injustice of suffering, and that the book (at least if I did it right) is more an exploration of the variety of responses to suffering than an argument in favor of one over another.
2. I do not believe the job of a novelist is to thrust his or her belief system upon a reader.