Is belief (or disbelief) in God even a choice?

There won’t be any spoilers here from me but I just watched Promethus again for the second time. In the movie there’s a line where a scientist (who happens to be Christian) is asked why she believes her long range scans and theories will prove accurate. Her response was: “That is what I choose to believe.”

I also read a comment in another thread that said: “Humans do not have the answer whether God actually exists or not, until then just pick what you believe ”

However, I call bull**** on both of these claims.

You can’t choose to believe in a thing or not, can you? For example, if you are a theist, can you simply choose not to believe in God on a whim? I doubt you can. Or if you happen to be atheist or agnostic, do you have the power to choose to believe in God on a whim?

Why do people (including the writers of various holy books) assume that it is a choice whether or not they believe?