The elder Zosima looms over the novel as its wise man, guru, and moral center.
Well, maybe not exactly "looms." Just like Yoda in Star Wars or Mr. Miyagi from The Karate Kid, Zosima is a modest figure. He's striking in his mild benevolence and humble manner. Despite all the special treatment he receives as the in-house celebrity at the monastery, Zosima remains down-to-earth in the most literal sense of the word, as he exhorts everyone to "kiss the earth" when they pray.
In his earthiness, Zosima practices what he preaches. And he preaches a lot, as the novel devotes a major section purely to his lectures – or "homilies" – and to his life story. In contrast to some of the more austere monks at the monastery, Zosima preaches a version of Christianity that emphasizes God's love as a force that embraces and animates all living things. Thus, for Zosima, a monk's duty is to, well, spread the love.
Perhaps the greatest responsibility for the monk is to spread the Russian Orthodox religion to the masses, who are overwhelmingly rural and poor, in the face of the rise of modern, "Western" secular values. So instead of abstruse, theological language, Zosima lectures through simple fables and digressions on the beauty of nature that often give a nod to Russian folk wisdom.
This earthiness makes him unpopular with some of the stricter monks at the monastery, who see his views as almost blasphemous in their emphasis on a loving God over a more morally domineering figure. But Zosima's view of the world isn't all sunshine and lollipops. Zosima is actually a realist, who recognizes the immense pain, suffering, and evil that exists in the world. He doesn't want religion to be an escape from the world, nor should the monk's solitude at the monastery be confused with such an escape. The challenge is to find all the beauty that Zosima celebrates within the world, despite its cruelty, injustice, and misery.
The novel shores up Zosima's realism by giving us an extensive chronicle of his life before he became a monk. While his affinities with Alyosha are obvious, as a young man, Zinovy (his given name) actually bears a lot of resemblance to Dmitri Karamazov, the most earthly of the earthly Karamazovs. Like Dmitri, Zinovy was a dashing young officer popular with women and downright cruel to his servant. In a critical duel scene, however, Zinovy experiences a moment of ecstasy while contemplating nature and lets his competitor win. This gesture – of freely choosing to lose – disproves Ivan's Grand Inquisitor, who claims that men are essentially weak, servile, and prisoners to their worldly desires. Zinovy's gradual transformation into Zosima signals the potential for all souls, even the most seemingly irredeemable ones, for redemption.
In the novel, the stench of Zosima's corpse causes a great scandal, because everyone expected it to smell Febreze-fresh, like a saint's. Zosima's stench only confirms his teachings: he is no otherworldly figure; he is firmly grounded in this world.