Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. (John 12:24)
The epigraph echoes the elder Zosima's teachings. He cites this particular passage from the New Testament's Gospel of John to suggest that suffering should not be the cause of our rejection of God, but an avenue into faith (6.1.14). In other words, suffering – particularly the suffering of innocents – may cause us to doubt the existence of a God who is just and all-powerful. But Zosima argues that suffering is necessary; it is the "seed" that can produce the "fruit" of a greater, a more robust faith. Through suffering we lose our pride and conceit; we become humble, and, in our humility, we are able to empathize with all human beings because we no longer consider ourselves superior to them. This empathy, or love, as Zosima stresses, connects us to the greater mystery of God's love.
In some sense, the novel is a test of what happens when suffering is sown in the fields of skepticism or faith, to stick to the gardening metaphor. If you are a skeptic like Ivan, suffering results in madness. If you are a man of faith, as Dmitri becomes at the end of the novel, suffering is a source of spiritual strength and regeneration.