Revelation 21:8

8  But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.

In other words, those who do not come to Jesus for forgiveness will retain their sins. As a result, they will have their part in the Lake of Fire. Being cast in the Lake of Fire is the second death. (The first death is death of the body of flesh.) I wondered why it said the fearful would get this fate with the rest of these characters, until I understood that it is talking about those that feared the death of the body more than the death of the soul like those that received the mark of the Beast to save their physical lives.