I am sure this person is being convicted by the Holy Spirit, and is fighting it. Or they are complacent with their sinful lifestyle, or some Christian may had hurt them deeply in one point of their life, or brain washed by some person they respect very much, and have believed that lie from hell. Remember God loves us even when we were still sinners. If I were you I would not condemn them, but pray for them, for the Lord to heal them from their unbelief, their hurt, or their sinful life. For we don't want to destroy whatever work God may be doing on them already that we do not know of.