Which word can be used to describe moralizing in a pompous manner?

The word "sententious" is indeed the most fitting choice for describing moralizing in a pompous manner. This term is derived from "sententia," which means "a saying" or "a thought." In modern usage, "sententious" refers to a manner of speaking that is moralizing or self-righteous, often in a way that is considered overly formal or lecturing. It suggests a tendency to express opinions or judgments in a preachy or pompously aphoristic style, indicating a type of discourse that may prioritize moral lessons over genuine insight or connection.

In contrast, the other options do not convey the same connotation. "Recreant" describes someone who is cowardly or unfaithful, "sidereal" pertains to the stars or celestial bodies, and "recumbent" refers to a position of lying down or resting. None of these terms captures the essence of moralizing in a pompous way, making "sententious" the clear choice for this particular context.