Unlike religion, science makes a simple and rational case about what we should do with our lives, and despite the gray matter that resides inside our skulls, we have evolved from lesser organisms to fulfill some very simple duties:
1. We eat.
2. We sleep.
3. We fuck.
4. We shit.
5. We have kids.
6. We die.
After several million years of evolution, humanity's basic role is to fulfill those six biological functions in spite of whatever beliefs (or unbeliefs, in the case of Atheists like us) we might have.
Of course, there's more to living your life than just doing those six functions. We have to do things like work, pay our bills, and do everything we can to improve the quality of our lives.
Life becomes much more simple when you realize that you are genetically programmed to fulfill those six functions whether you like it, or not. In essence, it is our job to fulfill these roles as our genes intended. However, it doesn't mean that we can run around and act irresponsibly. We're still bound by the laws, customs, and social norms of society.
On one hand, we are born to do those six things; on the other hand, what you do with the rest of your life is completely up to you, and once we understand why we're here and what we were born to do, we can move on to the more important things, like making the most out of the rest of our lives.