Atheists... what would it take?

Miller and Urey did the experiment, based on Oparin's theory of primitive Earth's atmosphere. By charging electrodes through the air containing the aforementioned substances (hydrogen) and then having it pass through a condenser, organic molecules formed, which then, combined with macrosomethings, could have created the first forms of life.

The molecules in the air would have mostly been present while Earth was forming. The Earth was formed over a 400 million year period (estimated) while it was constantly erupting and colliding with meteorites to combine and form a larger mass.