To what extent does race, caste and religion become important in Roy's The God of Small Things?The themes of the novel.
The elements you have highlighted are inextricably intertwined with the themes of the novel. Note how one of the central plot elements, the affair between Ammu and Velutha, breaks so many taboos. One of the interesting stylistic aspects of Roy's writing in this great novel is her use of repetition of set phrases. One of these phrases that is echoed throughout the novel is first found at the end of Chapter 1 when the narrator discusses when "it all began" and comes up with a number of different responses, finally concluding with the statement:
That it really began in the days when the Love Laws were made. The laws that lay down who should be loved, and how.
And how much.
Note how this important theme relates deeply to your question. This novel concerns the transgressing of these "Love Laws" that are of course built around race, caste and religion, and how these acts of transgression are censured and punished by society at large, whose job it is to maintain those love laws. Thus it is the father of Velutha that actually betrays his son, because he is so shocked and disgusted at his son having a relationship with an upper-caste woman. However, we would be wrong to focus solely on Velutha and Ammu, for Estha and Rahel, in their incestuous relationship, equally trangress these "Love Laws."