I've put some more thought into this, and it seems to me that the main issue is the commitment level of the people in 1 Kings 18. Ahab and the prophets of Baal and Asherah were committed to idolatry. Apparently they had made an ultimate, final decision to serve false gods. Elijah had made an ultimate, final decision to serve the one true God. The people who fell on their faces before God had been uncommitted to any god prior to their experience with God's power. I believe that when a person experiences the special, illuminating conviction of the Holy Spirit, he eventually has to make an ultimate, final choice about whom he will serve as his lord. Elect non-Christians will make an ultimate, final decision to surrender their lives to Jesus in repentance and faith. Some non-elect non-Christians will also experience the special, illuminating conviction of the Holy Spirit, and they will make an ultimate, final decision to reject Jesus. Such a rejection is an unpardonable sin (John 12:48; Hebrews 10:29).