Dostoevsky draws on religious and folk archetypes to give an allegorical depth to his novel. It's a way to show how a common trove of cultural meaning – such as religion – connects to everyday life.
Perhaps the three most important archetypes are the devil, the wise man, and the holy fool. While the devil, or Satan, is often represented as a towering, threatening figure of evil and terror – or, as in Milton's Paradise Lost , a Romantic, anti-establishment anti-hero – Dostoevsky's devils are decidedly less intimidating. The frailty of Ivan's Grand Inquisitor, and the shabbiness of the devil who meets Ivan in his room, and even Smerdyakov's sickly physiognomy, all indicate how evil lacks the vitality that comes from faith and love.
The two actual fools in the novel – Father Ferapont and Stinking Lizaveta – are at the margins of society, but still have an important place in it: Father Ferapont is honored by the other monks, and Stinking Lizaveta is cared for by the townspeople. These figures on the margins are revered or at least respected for being otherworldly, so intimately connected to the divine order that they seem mad.
The wise man may seem to be the opposite of the holy fool, but they are actually quite intimately connected. The truly wise man often appears to be a fool to others; his otherworldly wisdom doesn't compute with the worldly concerns of most people. The characters' reactions to the two "wise men" of the novel – Zosima and Alyosha – are usually uninhibited laughter and joy. When Zosima – in his early life as the young officer Zukovy – concedes the duel and explains his spiritual enlightenment to others, he is greeted by puzzlement and joy. Similarly, characters automatically react to Alyosha with friendship and confidence, if not joy. Zosima and Alyosha's friendliness bears out the thesis presented in the novel that "a man must suddenly set an example, and draw the soul from its isolation for an act of brotherly communion, though it be with the rank of holy fool" (6.2.d).