Answer:
The most important concept to remember is, you can do nothing that will make them feel better.
It takes time to get past the feeling of loss. Religious assurances does nothing for the bereaved, but sometimes the people doing the talking feel better.
Anything you say will seem like the babbling of a friend who has become neurotic... There are times, especially when a family is deeply embedded in some type of religious extremism, that religious assurances MAY be appreciated, but in general, if the family is already involved in that, your words won't make much difference anyway.
Enough of what NOT to do, but there are some things that you can do.
When the moment is right, talk about one of your memories of the deceased, especially a memory that is positive or mildly amusing; nothing excessive or crude, the guiding principle should be, "will it be a good memory for the bereaved", and "will it be generally uplifting"?
Funerals, wakes, viewings and other events when the family is obviously in pain should NOT be a time for you to discuss your fears, but you should make those times about the deceased.
All too often, nobody outside of the family wants to talk about the person who has died; they often act as though the deceased never existed.