Some atheists say they'd believe in God if he arranged the stars to spell out John 3:16–or something like that. Indeed, this has become an atheist trope. Theodore Drange, Jerry Coyne, Evan Fales, Matt McCormick, and Keith Parsons, among others, have used that basic illustration.
Of course, it's a facetious illustration. Because so many atheists have intellectual contempt for Christianity, they easily succumb to thinking there's a quick and easy way to dismiss it. As a result, they resort to glib, shortsighted examples.
The problem with the skywriting example is that it conflicts with how many atheists define a miracle. Taking their cue from Hume, many atheists define a miracle as a violation of natural law.
But on the face of it, a conjunction of starry objects (e.g. stars, comets) to spell out John 3:16 doesn't violate the laws of physics. Rather, it fits the definition of a coincidence miracle.
In principle, God could plan the history of the universe so that in the year 2000 AD (or whenever), there's an alignment of starry objects spelling out John 3:16. That might be in the works from the time of the Big Bang. God could work through natural processes to arrive at that result.
It doesn't require the stars to suddenly rearrange themselves. It only requires a combination of starry objects of absolute or apparent magnitude to spell out that message. It doesn't require any star to change course. This physical conjunction could be physically predetermined from the time of the Big Bang. A delayed reaction.
(I'm not saying I subscribe to the Big Bang–just using that frame of reference for convenience.)