DOUBT
The play is saturated with a sense of non-commitment, doubt and failure of faith.
A good example occurs in the theological dialogue between Marcellus and Horatio in Act One. In response to Marcellus' non-scriptural account of the "the bird of dawning singing all night long" to signal the season of Christ's birth, Horatio responds, "I do in part believe it." I.i.165. Marcellus' tale of a non-scriptural sign, and the partial belief of Horatio, are subtle examples of the doubtfulness and weakness of faith that permeates the play.
Another example is Hamlet's doubts about the reality of the afterlife. Even his reason for not committing suicide -- the terror of the afterlife -- is mentioned dubiously: Hamlet is not really sure about how divine justice would deal with suicide, saying "what dreams may come . . . must give us pause." III.i.66-68.