Most if not all theists believe atheism to be a choice. To them it is something you can choose to accept or reject. Therefore, to the typical theist, if you become an atheist, you have chosen to be one. If you leave atheism, then you have chosen to leave it. While this second argument is most likely true. The person who decides to abandon atheism for theism has most likely made a choice. The person who becomes a theist has usually made a choice. [except in the case of young children raised as theists, who are almost always too young to make this choice.] However, is it true that a theist who converts [or more accurately "deconverts"] to atheism, really chosen to be an atheist? I don't think so.
For me, I certainly did not have a choice in becoming an atheist.
Although I am now quite happy as an atheist... when I first lost my faith, I wanted SOO BADLY to go back to it.
Being raised as a typical theist, I held the erronious belief that if I simply went to church and prayed hard enough, then Jesus would answer me and turn me back to him and away from my sin of "rejecting him." - Amazingly, I was so badly brainwashed at this time, that I didn't realize that I didn't choose to become an atheist, that it just happened, I thought that "I was being sinful and turning away from God, or God was "testing me."
So... I went to church and prayed, and begged, and sang praises, and through my heart and "soul" into trying desperately to find my way back to Jesus.
Certainly I tried hard enough, but the "light of God" never touched me, I was outcast.
So, does one "choose" atheism...
I doubt it.