Falsehood (i.e., lying).
Lying that springs from delusion (e.g., you are pretending to be something you are not) is wrong because it necessarily takes you from the path. Beyond this, neither honesty nor lying are good or bad in and of themselves. Example: Your mother just got her hair done for a big party. She loves it and asks you how it looks. You think it's awful. Is it right to be honest or to lie?
Believers do not accept science as valid proof and non-believers do not accept faith as valid proof, so this debate cannot be settled beyond yourself. For me, I do not believe in god(s), but it's certainly possible I could be wrong. However, Zen Buddhist considers fretting over a question that cannot really be solved as cluttering your mind. It's delusion that just takes your from the path and solves nothing. Frankly, I think my realization that this question is irrelevant (and then clearing my mind of any quibbling around it) has propelled me forward tremendously. So you see that the existence of god is not an essential question for a Zen Buddhist to answer.