In Act 1, sc. 5, line 142, Hamlet says, "It is an honest ghost - that let me tell you." Hamlet is telling Horatio that the spirit he just saw- that of his father - was the true spirit of his father, following the Catholic view that ghosts could be the spirits of people who were in Purgatory. The ghost tells Hamlet that he, indeed, is the spirit of his father doomed to Purgatory for some time because he did not get a chance to be absolved of his sins since he was murdered (Act1, sc. 5, ll. 10-14). Hamlet seems convinced when he is talking to Horatio and Marcellus in this first act that the ghost he saw was telling him the truth about what happened and when the ghost asks Hamlet to seek revenge against Claudius, Hamlet quickly agrees to do so. In Act 2, sc. 2, we know that a considerable amount of time has passed since Act 1 because Laertes has had enough time to go back to France and to write home for more money. Hamlet still has done nothing and his frustration with himself and his lack of action is shown in the soliloquy that ends Act 2, sc. 2. He says, in lines 607-612 that he knows the spirit he saw may have actually been a demon trying to lure him to hell, which was the Protestant view of spirits at that time. He has changed his opinion about the ghost. Now he isn't so sure that it "is an honest ghost". That is the irony.