The type of megaprojects being discussed in this thread are those that span lifetimes. Getting a man to the moon — without dismissing its huge impact, it was definitely a great work of mankind — was still done within a single lifetime.

I posed Proxima Centauri simply b/c it wasn't religious, and perhaps palatable as something humanity should undertake, but not feasible within a lifetime, as a counter-example to show that not all lifetime-spanning projects are worthless.