I wouldn't expect most people who have abandoned churchgoing to refer to themselves as atheists, though. That's a step too far for most people. Many people who do not practice a religion still think of themselves as somehow religious or "spiritual," whatever that means. They just don't think about it much, nor do they actually participate in any sort of worship.
Deciding that you don't believe any deities exist requires some thinking and reasoning. That's hard work. The conservatism bias keeps the brain hanging on to old ideas, even when presented with a new concept. It's easier to do that than to accept an idea that is completely the reverse of your old beliefs. Since there's no identifiable major benefit connected with disbelief, there's no reason to abandon deeply-ingrained beliefs, even if you no longer think about them much.
However, religion is a learned thing. Once adults stop participating in religious activities, it's likely that their children will not experience nearly the same level of presentation of religious ideas that those adults did. After all, most of us were taken to church by our parents to be taught the same religion they believed.
So, it's likely that the offspring of church-leavers will be prone to consider themselves atheists when they begin thinking about such things. Without the regular teaching of religion as children, they are far less likely to have religious beliefs deeply embedded in their developing brains.
For religious beliefs to take hold, "You have to be carefully taught."
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