It depends on the nature of your faith beforehand, and your lack of faith afterwards. If, as a child, you were just dragged along to church by your parents at

It depends on the nature of your faith beforehand, and your lack of faith afterwards. If, as a child, you were just dragged along to church by your parents at Christmas and Easter, then when you grow up and realise you don't believe, so stop going, that's one thing, and there's nothing really "taken up" about it. If you were a regular adult churchgoer of your own volition, perhaps with a lay position within the church, or even part of the clergy, then apostasy, is a strong act that might be called "taking up" your later position, even if the later position is a lack of one. Similarly, if you become a normal atheist who simply doesn't believe in any gods, then there's nothing to take up. However, if you become an evangelical, proselytising atheist like Richard Dawkins, then you've certainly taken it up.