Should our understanding of the word religion be complete, or incomplete? Naturally, the answer is complete. To have a complete understanding of a concept is to understand the varying connotation of particular terms.
Religion is defined the way you present it, but it is also defined as simply:
“A pursuit or interest followed with great devotion.”
This definition exactly describes atheism. Frankly, I would say my definition is possibly better: a system of belief establishing worldview, and the behavior patterns derived from such belief. Although I would insert something about devotion or strong belief to make it even better.
As I've noted ... on the m-w.com dictionary website, the 2nd & 4th definitions of religion apply to atheism.
You go on to claim that ""This definition exactly describes atheism"
I claim it does not.
Most of the atheists I know barely think or talk about it. Thinking six friends who I know are atheists their "interest followed with great devotion" are music, martial arts, their dog, civil war reenactment and beer. In fact they just describe their lack of belief as "I don't believe in God" and/or "I'm not religious"
Your argument is akin to saying "failure to participate in bowling, is a form of bowling."
Your second alternative definition is much better: a system of belief establishing worldview, and the behavior patterns derived from such belief.
But again, this is not the core of religion, it's sort of using the word to describe an effect of religion. Using this definition being a "Conservative" "Libertarian" or "Socialist" would be a religion. But, again that's an unconventional use of the word.
Can we really generalize about the "behavior patterns derived" from Christianity? With almost 2 million Christians, I think it's hard to generalize like that.
I think a lot of people's behavior is influenced by religion, and for some it's the overwhelmingly most important influence. But for a lot of people it's not the most important and things like culture, politics, social networks, and education have more influence on their behavior than the religion they profess.