Quotes provided are from Plato’s Republic translated by Allan Bloom copyright 1968.
The question of piety is raised immediately in the Republic, “I went down to the Piraeus yesterday with Glaucon, son of Ariston, to pray to the goddess; and, at the same time, I wanted to observe how they would put on the festival..” (327A 1-2). This is clear that from the outset Socrates descends into the Piraeus for a religious cause, to pray to the goddess and observe her festival. Regardless of the fact that he has ulterior motives for going to the Piraeus (namely to test Glaucon), it is argued from the outset that he is going to fulfill his pious obligations.
Next we arrive at the house of Cephalous, who, after discussing justice with Socrates leaves the discussion to attend to his sacrifices. It is clear from this point that piety is an essential aspect of human life; Cephalous is near death and therefore must attend to those things which most concern death. Also apparent is that Cephalous has no stomach for philosophy, as after he leaves the discussion of justice becomes philosophic. Does this mean that piety is in conflict with philosophy? No it does not. Piety is an essential part of the comprehensive human activity (polis).
How is this clear? Socrates begins book three by saying, “About gods, then’ I said, ‘such, it seems, are the things that should and should not be heard, from childhood on, by men who would honor gods and ancestors…'”(386a 1-3). The teaching of the gods is an important aspect in the education of the citizens. But there must be a proper method of teaching the gods, and it is in teaching them in a way that they reflect the desired formation of the citizens. But this quote also brings up another important aspect, that being the honor of ancestors. It is clear that the relationship between the gods and the ancestors is close. Thus to honor the gods is in a way to honor one’s ancestors. What is the best way in which to honor one’s ancestors? To act according to how they would want one to live is a possibility. The legislator must aim at educating his citizens to honor the city, one of the ways to do this is to tie the ancestral to the city. By behaving as the gods behave, one is imitating his ancestors as they too were taught in a like fashion. Honoring the ancestors also comes in upholding the city that they helped to create, and again in order to form your citizens properly you must teach them the gods.
Piety within the Republic comes on three levels, the beginning of the dialog is centered on religion and religious piety. Next we find that religious piety is an important aspect to the person, specifically the old and close to death. Third, in order to properly form your citizens the legislator must take care to teach the gods in a manner which train the citizens to be a certain way. Piety must be subordinated to philosophy, specifically political philosophy which is aimed at the comprehensive human good: happiness.