I thank God for His Word. There are some difficult passages in the Scriptures that would cause one to stop and think about the whole "guaranteed salvation no matter what" premise. I know it can depend on how you approach the passage also.
One such place for me is found in Matthew 13:41-13:42. It seems to be speaking of Christians that continue in sin and at the end of this age being excluded from salvation. If anyone has studied this passage at length and cares to offer any illumination upon the context, feel free to do so.