Atheism and agnosticism are not mutually exclusive. One can be both atheist and agnostic, or neither.
If Christianity is not true, it lays out a false purpose for life. As an atheist, who was a Christian for 22 years and attended a Baptist university for part of my undergrad, I can testify that my de-conversion ultimately gave my life more purpose, meaning, and direction.
The irony of your whole argument is that if you’re wrong, then you have no morality. Blindly doing what you are told is not exercising morality. Most atheists that I have met who used to be Christians would say that moral decisions became a lot simpler after their de-conversion.
Atheists do not, “not believe in anything.” Atheists believe in the same things you do for the most part, with the exception of a god. Atheism is not a “belief system” or a “religion” for the same reason that no believing in ghosts is either of those things. If you are looking for atheistic worldviews: humanism, naturalism, and skepticism are all good options. However, these are still not religions because they have no tenets, dogma, ect. They are a lens through which the world is viewed. An atheistic religion would be something like Buddhism.
Imagine that you asked someone how a vending machine worked and they told you “a team of gremlins are inside.” When I hear someone say that, “the earth was created 6-10k years ago”, or “the universe was created by God,” I hear “gremlins in the vending machine.” I would much prefer not knowing something than believing something false.