If someone wants to convert an atheist to Christianity (or anything else), they might start by first convincing them that a god (pick a god, any god) exists.

As an atheist of some 55 years, I've thought about what it would take to convince me that there is indeed a god, Christian or otherwise. I just need to see, or to see incontrovertible evidence of, a god or their duly appointed agent on earth instantaneously restoring to full function a human being's amputated or never-existing arm or leg. That is a feat that I can imagine no current or future medical technology ever being able to do. If a god could do it, I'd certainly believe in that god.

That seems like a relatively minor miracle for a deity who created the entire universe to perform, doesn't it? But think of the impact it would have on humanity!