 Because according to your worldview, every single thing you're saying, everything you're doing, everything you think, the fact that you're an atheist is something just absolutely determined by material causes. It's the way the particles have interacted. It's the way the electrical chemical processes in your brain are transpiring. You're just a machine. And you're saying there is no God, there is no soul. And guess what? I'm a machine, according to you. I'm a machine and I'm saying there is a soul and Christianity is true.