The main theme of a literary work is often the one that has the widest general applicability. In The Castle, the theme of the individual’s struggle to fit into society, even as he opposes it, is most likely the primary theme. Scholars have debated the meaning of the Castle itself, as well as the nature of K.’s quest. Some interpreters tend to see it as a religious work, with the Castle representing God or heaven. Others consider it as a secular, existentialist treatise, with the Castle having a more abstract significance, such as the meaning of life.

Pagination will vary depending on the edition. The quotes provided here correspond to the 1922 Muir translation.

On pages 20--21, K. is looking at the Castle, mentally comparing a tower he sees to the church tower in his own hometown. He meets a teacher coming out of the school. The teacher immediately accuses him of not liking the Castle because “Strangers never do.” K. admits his anxiety about fitting in, noting that he feels lonely because he won’t fit in with “the peasants nor . . . the Castle.” The teacher replies that there is no difference between them. This passage highlights K.’s difference and alienation from society.

On pages 59--61, K.’s relationship with Frieda finally comes to fruition in the bar, but he feels anxious despite his happiness:

K. was haunted by the feeling he was losing himself or wandering into a strange country, farther than ever man had wandered, a country so strange that not even the air had anything in common with his native air.

Pages 96--98 (rather than 100 and on in this edition), are concerned with K.’s anxieties about being summoned to the Castle, contrasted with the threat of being thrown out, which seems imminent. Conversing with his landlord, he declares, “So the only remaining conclusion,… is that everything is uncertain and insoluble, including my being thrown out.”

As you've already stated, Kafka's The Castle contains many different themes, and, as a result, it can be interpreted in several different ways. While themes of bureaucratic absurdity, alienation, and political abuse are common in the novel and in Kafka's other works, The Castle can best be interpreted as a religious allegory for man's inability to communicate with God.

The Castle is, at first, invisible to K. When he arrives in the village, his goal is to reach the castle, but he is unable to; he cannot even see it, yet he believes it exists, that it is within reach, and that his goals can simply be met. Yet, throughout the novel, as K. attempts to communicate with the castle, he only finds silence and a complete lack of answers.

At one point in the novel, after K. asks for permission to ask the landlady a question, she says nothing in return. K. then takes this as a sign that she does not wish to be questioned, to which the landlady replies,

"You misinterpret everything, even the silence" (60).

This idea of silence being misinterpreted goes hand-in-hand with the religious allegory. When one speaks to God, whether in prayer or asking questions, they are offered silence in return. How one chooses to interpret this silence, however, can vary dramatically. When K. looks for answers, he is often met with silence. Does this mean that those in question are choosing not to answer, that they are unable to answer, or something else entirely? It's up to K.'s—and the reader's—interpretation.

The Castle serves as a metaphor for God since no matter how hard K. attempts to reach it or speak to it he is unable. The Castle doesn't answer and is only able to communicate with officials who themselves admittedly know very little about the Castle's intentions. This is a metaphor for how one cannot converse with God but only with representatives (priests, rabbis, pastors, et cetera) who cannot claim to know everything that God knows.

In The Castle, K. is completely at the Castle's mercy. He cannot speak with it, he cannot truly see it, and he cannot reach it, yet he continues to try, completely in vain, to reach his goal.

Works by Kafka can always be interpreted in a number of ways. One way to interpret "The Castle" is to see it as a representation of social norms. The protagonist, Joseph K. attempts to be a part of those social norms. The main theme of the story is mankind's constant struggle to belong somewhere. In this particular instance, Joseph K is not accepted or rejected. He exists in a state of limbo, a kind of in between where his belonging is always pending. This idea is closely tied with the idea of "liminality", where an individual's identity is neither fully one nor fully the other, but an empty in between state.