Symbolism, Imagery, Allegory
There's a lot of Old Testament language in the novel and Isaac is no exception. Remember Isaac? The young man in the Book of Genesis whose father almost slaughtered him on an altar at God's command? Lucky for him, God relented and provided a ram to be sacrificed instead. Not lucky for the ram.
Our Isaac also feels like a victim of his patrimony, but he knows he has to save himself. He's "declining the altar." He gets out while he can. By naming our guy Isaac, Faulkner allows us to see him as a metaphor for escaping a destiny that others determine for us.