Yes, it is a belief. They cannot prove or demonstrate it ... nobody knows what happened in the early universe, they only suppose. They do rationally extrapolate back in time, to a small fraction of a second of the Big Bang, because of particle physics. To presuppose a Creator, is to irrationally extrapolate back in time. But that is like extrapolating from a folded newspaper boat to a giant ship. Their belief is rational, but they also deny it is a belief. You agree that they are rational, but in denial. I deny both rationality and belief, I cast ridicule upon extrapolation back in time. If you can't demonstrate something, here and now, you have nothing. There is nothing to believe and nothing to rationalize. The Big Bang itself, cannot be demonstrated .. though they always hope that super science will prove them right, eventually. Like the ugly girl who thinks every year, she will be invited to the Prom ;-) It will never happen, physics knows now, to have ultimate knowledge ... infinite power is required. This is unobtainable, forever. Seeing before the 3.5 K radiation is also probably impossible, the constitution of the universe at that time was uniformly chaotic. If a naturalist moves far enough, he becomes a theist and vice versa.