Which god did you have in mind? The framing of the question contrasts the different view the believer tends to have compared to how an atheist or agnostic approaches the subject.
I cannot see how it is even slightly "inconsistent" for an atheist to argue that a particular god doesn't exist, though it would seem the questioner is trying to beg the question with the phrase "work against his existence" thereby embedding the reality of the god as an assumption.
Did you ever believe in a god, and if so, why did you change your mind? What hope do you have when you and your loved ones die and why are you OK with that?