Thanks. I appreciate the correction.
Oedipus at Colonus is even hazier in my mind than Oedipus Tyrannos, but I'll take your word for it. I think that's right, though, that perhaps Oedipus at Colonus more explicitly expresses some of the theological ideas that underpin Sophocles' worldview -- especially the ideas that there is a necessity that governs the fates of individuals, that that necessity is insurmountable and human beings have no power to change it because the very definition of necessity in this case is that it is what cannot be otherwise, and that that necessity is the will of the gods (especially Zeus).
I think that Sophocles' view on necessity isn't too distant from the Stoic view which is expressed in a metaphor that compares how humans relate to fate to a dog being dragged by a cart. The cart is going to move forward, and the dog can resist it, or he can accept it and move with it, but either way, it's going to pull him forward.
Sophocles was of the view that fate operates in a similar way. No matter what a person does or wants, fate will govern their lives. Therefore, the best that humans can do is be aware of it, recognize the power that fate has over human life and the limited ability of human beings to change it, and accept it. But the worst thing to do is try to resist it, because it's impossible, and ultimately leads to the ruin of those who are impudent enough to resist (or disquietude and perturbation, in the case of the Stoics).
In that schema, impiety is defined as the failure of an individual to recognize how fate operates in his/her own life. It is an insult to the gods to fail to recognize the specific fate that one has been allotted, and to know that is just what self-knowledge is: to know one's fate is to know oneself. Thus, self-knowledge (which takes the form of a virtue, sophrosune) and fate are closely related, and the absence of self-knowledge is a vice.
(Sorry if I'm being obnoxious by writing all this out. It's been a while since I've had to think about it and it's a pleasure to be able to do so again.)