> Let's imagine the Christian god doesnt't exist. How does that logically lead to nothing mattering?

The entire book of Ecclesiastes is about this. I wouldn't presume to summarize it all in a comment box, but it does cover a lot of grounds in different approaches to the meaning of life. It claims none of them really work all that well.

And no, it doesn't say, "But you die and go to heaven or hell anyway" over and over. It's not very supernatural at all, actually.