Proving a negative is nearly always a futile effort. Reasonable secularists know better than to try, which means the ones you hear being noisy about it are generally the unreasonable ones. I don't lack faith in God because I can prove there isn't one, the same way I don't lack faith in Bigfoot, pink unicorns, or the alien invasion fleet that's going to land tomorrow because I can prove they don't exist. I lack faith in any of them because I've seen no credible evidence to suggest they do, and have no reason to prefer any of the wonderful ideas people can dream up to any of the others.