I'm sure you're right. Some people claim to be Christian but still presume to take their sins lightly. However, I think it's kind of strange they would do this when Paul the Apostle tells us in the letter to the Philippians (3:12-16) that even though he knew God loved him, he didn't assume he had yet fully attained a status of salvation, or that he was yet perfected. In fact, just after Paul says this about himself, he then warns the Philippians to be careful because he knew there were some individuals among them who, although perhaps baptized and associating in the church, weren't really following God's Will:
For many walk, of whom I have told you often, and now tell you even weeping, that they are the enemies of the cross of Christ... Philippians 3:18
So, you're right; people who claim to be Christian, even those who think they're "true believers," need to be careful to examine their lives and hearts since apparently it appears that some will actually come up short on the Day of Judgment.