Maybe I do only react and counterreact. If so, and if someone tells me the Final Fact also only reacts and counterreacts, then I might as well conclude the FF is also a Person.
But an atheist will say that the structure of my thinking organ, my brain, is what makes the difference between me being a person, and the FF (usually Nature) not being a Person.
It is not that my brain is more complex than the total field of Nature–my brain certainly is not! But it does have certain arrangements that are complex, in specific ways, distinct from other portions of Nature (such as, for instance, this book you are reading).
Can that possibly make a difference?
One type of atheist will strenuously claim it makes all the difference between reaction and action.
The other type of atheist will strenuously claim it cannot possibly do any such thing–but it does make some other crucial kinds of difference.
You may notice this has parallels with the question of computer AI.