 Verse 18 speaks about the spiritual darkness which unbelievers walk in, their ignorance and their blindness. They have their understanding darkened. They are alienated or estranged from God, from the life of God through ignorance that is in them. Many times an unbelieving person who may seem hostile towards the truth, they're really simply uninformed. They don't know. They're walking as a blind person, and they deserve our pity and love, not our anger and hatred back towards them.