The definition of Omega includes him being completely honest and trustworthy. He wouldn't tell you "I will make your afterlife better" unless he knew that there is an afterlife (otherwise he couldn't make it better), just like he wouldn't say "the current Roman Emperor is bald". If he were to say instead "I will make your afterlife better, if you have one", I would keep operating on my current assumption that there is no such thing as an afterlife.

Oh, I almost forgot - what does it even mean to "believe in science"?