Specifically, what do you, who are Atheists, believe about why Theists are theists? For example, I hear quite often and see just as often statements like “you only believe in God/your God because of your parents/upbringing/where you were born. Break free!” Sure I suppose some, maybe even many religious adherents believe for the sole reason of life circumstances, but that certainly isn’t all or most — and it’s especially not the case for me.
As an Atheist who may have leveled this accusation in the past, what do you think it proves? In other words, ‘You only believe because of your upbringing, therefore ______.’ Can you fill in the blank?
I agree that it’s a good idea that you hold your core convictions based on the fruits of some investigation, but a conviction is true or false no matter the reason you came to believe it. It would only be the case that while the conviction is true, it wouldn’t be considered knowledge in the philosophical sense (justified true belief).
So, since the origin of a particular conviction is irrelevant to whether it’s true, why does this complaint still persist? Isn’t this accusation valid if applied to a person raised in an Atheist home? It doesn’t seem to have any value other than it’s rhetorical effect.
Sifting Reality 
The majority of religious adherents are a member of any given given sect or denomination because they were reared in it, hence the astronomical number of followers for a relatively few particular religions.
And well beyond the rhetorical effect, the most heavily weighted argument, is that when a child is reared in such a home with no ability to explore anything else, it is akin to indoctrination. I would ask how it is any different than being in a Chinese work camp and its propaganda machine.
And most of the non theists I know raise their children with critical thinking faculties and the freedom to explore. My 7 year old has no idea what an atheist is and we have let him explore what his friends do at Sunday School. He was unimpressed and found it “stupid”. He has never heard my wife or I frame christianity in this way. He came to this on his own. So we will not be indoctrinating him with non belief. I will push them to discover on their own though.
It cannot be said that this is the same dynamic in the Abrahamic religions. Those who question their parents beliefs are either coerced into believing or ostracized or in the case of Islam sometimes killed.
I believe that theists have quite varying reasons for being religious. I agree that religion itself is often transmitted culturally, and that many times people are indoctrinated into a religion because it is the religion of their parents- but I wouldn’t say that this is the only possible reason for people becoming theists. It certainly correlates nicely with the revelation of choice for those who do profess a faith.
The rest of this post uses some specious logic that you disavow yourself in the first sentence of the third paragraph but then go on to assert as a reasonable argument. It is true that people can come out on the side of truth without critically examining a claim, but this is neither salient nor remotely advisable.
I could, if I so chose, create an “Epistemological Roulette Wheel” where the one correct answer was interspersed with 37 wrong answers and use it to choose what I believe about any given premise. If I did this, I would have a 2.6% chance of being correct on any one question. Is this something I would consider advisable? Should I consider it advisable?
What you suggest when you say:
is that knowledge is essentially irrelevant to the likelihood that your conclusions are correct. That is more than specious, it is outright false at face value. Ignorance doesn’t guarantee that you are wrong, but it certainly makes you far less likely to be right- in fact it does this in relative proportion to your level of ignorance. You are either abusing or ignorant of what the word “irrelevant” means. The origin of a conviction is more than just relevant, it is the best guarantor of accuracy available.
I’ll grant you that outside of a focused discussion on the origin of epistemologies the charge itself is flippant and not always relevant to the conversation at hand, but it is far from irrelevant or unfair.