"Soliloquy of the Spanish Cloister" takes place in a cloister, or the central garden of a religious monastery, and the speaker and the other main character, Brother Lawrence, are both monks. So it makes sense that religion should be a major theme. But what makes a good monk, in the mind of the speaker? Is it the little things, the details like how you polish your plate and how you drink your orange juice? Is it strict adherence to the letter of the law? Or does religion require something more than an outward, formal show of piety?
The speaker's religious practice concentrates solely on the formal, visible aspects of monastic life; he obeys the letter, rather than the spirit, of the law.
The speaker's obsession with precise, dogmatic interpretation of monastic law accounts for his repeated references to heretics, whose disregard for accepted doctrine makes them, in his mind, the worst of sinners.