Jordan's stream-of-consciousness thoughts serve as readers' primary narration. These thoughts are scattered, reflecting Jordan's subconscious internal monologue rather than an objective presentation of events. In this regard, he's an unreliable narrator. Note the following quote from Chapter 18:
They were Communists and they were disciplinarians. The discipline that they would enforce would make good troops. Lister was murderous in discipline. He was a true fanatic and he had the complete Spanish lack of respect for life.
Jordan states that Lister's discipline will make good troops, then explains that Lister possesses the "complete Spanish lack of respect for life" (a hyperbolic statement in and of itself). Clearly, a good leader and good disciplinarian would not encourage their troops to completely disregard the sanctity of human life. Readers should question this point of view, as Jordan often questions himself.
Authors often utilize an unreliable narrator as a means of calling readers' attention to their own thought processes. If the narrator's sole word cannot be implicitly trusted, readers must question their own interpretations of events, turning a critical and meta-narrative eye on how they respond to the narrator and the story itself. For Whom the Bell Tolls is not attempting mind games on its readers; rather, it is presenting clearly subjective narration in the hopes that readers will empathize with Jordan's uncertainty.