Symbolism, Imagery, Allegory
Not sure if you've heard, but the moon is a pretty big deal.
The moon is to mythology what Beyoncé is to pop music. There's a very simple reason why: That thing is always up there in the sky. Like Queen Bey, the moon is a constant part of our lives, and yet something we don't actually know first-hand. Just like Beyoncé has yet to invite us to a party at her house (the invitation probably got lost in the mail), we're also yet to go to the moon. And while at this point, some folks have visited that glowing white orb, when Verne wrote this book, no one had. The moon, then, represents uncharted territory. It's a last frontier.
To this end, the Gun Club has rejected the "ignorance of former ages" (6.1) in favor of a fact-based, scientific approach. In other words, while in the past people have worshipped the moon, what they're doing is different. In fact, the main reason they decide to go is simply to add the moon "to other thirty-six states which form […] the union." (2.23). To them, it's just another land to claim. That said, the fervor with which they work on this mission kind of looks like worshipping if you look at it in the right light—they give this project their all.
And this, we think, is kind of the point. As much as the members of the Gun Club fancy themselves scientists and objective observers, the reality is that they're pretty enamored with what they do. In setting their sites on the moon, they join a whole host of other people throughout human history who have stared into the sky and imagined what might be going on up there. They might believe nothing is impossible with science, but then again, religious participants usually feel the same way.