since David was open and above boards about his communications with his mother from his opening scene I didn't consider it a "secret"...
Well, of course we're wise to the ways of DS
- but at the outset at least the audience is supposed to follow Liz and Roger's belief that David is very troubled, Carolyn's opinion that he's nuts, and Julia's assessment that David has a psychosis in which he believes his mother has some sort of cyclical immortality (all of which Julia finds fascinating). But David's "secret" is that everything he claims about his mother really is true.
For most of the film, the only people who are possibly willing to believe that might be true are Barnabas and Vicki. Though, of course, both of them have good reason to possibly believe it.