I don't believe any true Christian can lose their salvation. God keeps His own. What I do believe is that professing (but not possessing) Christians or apostates are going to be shocked one day to find that they were not really saved (if they ever thought they were). These people that Revelation 22:18-19 are referring to are the lost - though they may have a form of godliness, they are tares among the wheat, wolves among the sheep.