Various things in the Bible come off as figurative when read through a modern lens, but were taken literally by the original audience. The idea of an eternal soul is one, that's what the NT is all about, that our souls will be resurrected to eternal life if we accept Jesus's sacrifice. Another one is demonic possession. It's easy to imagine that that's a symbol by someone overcome with rage, but to the ancients, demons were the literal explanation of that.