Translation: If you don't believe that the real world is but a pawn in a battle between competing supernatural personalities who only communicate to us via mental telepathy, then you'll be bereft of moral convictions.
That would be a very narrow way of putting it. To be more general, and more precise, I'd say that if you don't believe in a benevolent higher power of some form, you will likely fall for a darker or more mechanical world view. This world view will likely result in your stay on Earth being less fulfilling and happy than it could have been.
People who have never heard of the Bible or the Koran can be atheists. I'm sure there are some very happy Buddhists and animists out there.
As far as science being a religion, I apologize for the shorthand reference. Yes, you are right in that moral behavior can be scientifically proven to benefit humanity. That will never sell more than a fraction of the human race on the importance of morality.
My point is that faith in science may be enough of a guiding light for someone with no spiritual faith. We can pretend that if we build faster machines, larger cities, and invent more flavors of ice cream then we'll someday be happy. Many scientists take their causes to heart, and are fulfilled by them. But science is not an end unto itself. It simply allows us to understand and manipulate the world around us.
This can be a noble endeavor. It sure beats sitting in caves waiting for it to get warm again. I just don't think that people who put their faith in science alone, that is people who believe in nothing besides what can be proven, are getting the full experience that life has to offer.
To me, the only disadvantage to being an atheist is we don't get to believe that we'll live forever. OTOH, that does mean that this life is the only life we get, and so the principles that guide our actions are important in themselves because of the consequences to ourselves & others.