Well, I concede part of your point. Here's the first definition of religion from Wiktionary: "Belief in a spiritual or metaphysical reality (often including at least one deity), accompanied by practices or rituals pertaining to the belief." Notice the part in parentheses. So it comes down to what one can consider a "spiritual" reality, not necessarily involving a deity.

These days some of us atheists choose to practice what we call a "natural religion" based on scientific knowledge and the natural universe, but acknowledging that we humans construct our own "spiritual" meaning for our existence, and that we will probably never know everything about the universe and our existence in it. It's an open idea of religion, and we must somehow strive to build rules of ethics on it.