Plato (Πλάτων, Plátōn) was a philosopher in Classical Hellas who lived from 428/427 or 424/423 to 348/347 BC. He was the founder of the Academy in Athens, the first institution of higher learning in the Western world. He is widely considered the most pivotal figure in the development of philosophy, especially the Western tradition. Unlike nearly all of his philosophical contemporaries, Plato's entire œuvre is believed to have survived intact for over 2,400 years. I would like to quote from one of these works, namely 'Euthyphro'--named after one of two chracters in the writing (the other being Socrates)--today.

Euthyphro (Εὐθύφρων, Euthuphrōn) was written around 399–395 BC. It is a dialogue that occurs in the weeks before the trial of Socrates (399 BCE), for which Socrates and Euthyphro attempt to establish a definitive meaning for the word piety. Euthyphro of Prospalta (Εὑθύφρων Προσπάλτιος, Euthύphrōn Prospáltios) lived around 400 BC. He was an ancient Athenian religious prophet (mantis) whose ideas gave rise to the Euthyphro dilemma: is the pious loved by the Gods because it is pious, or is it pious because it is loved by the Gods? 

Piety, by definition, is reverence for the Gods or devout fulfillment of religious obligations, but as Plato has Socrates question: what does that actually mean? I'll let Euthyphro and Socrates answer--or attempt to, at least.

Socrates: Tell me what is the nature of [piety], and then I shall have a standard to which I may look, and by which I may measure actions, whether yours or those of any one else, and then I shall be able to say that such and such an action is pious, such another impious.
Euthyphro: Piety, then, is that which is dear to the gods, and impiety is that which is not dear to them.
Soc. Does not every man love that which he deems noble and just and good, and hate the opposite of them?
Euth. Very true.
Soc. But, as you say, people regard the same things, some as just and others as unjust,-about these they dispute; and so there arise wars and fightings among them.
Euth. Very true.
Soc. Then the same things are hated by the gods and loved by the gods, and are both hateful and dear to them?
Euth. True.
Soc. And upon this view the same things, Euthyphro, will be pious and also impious?
Euth. So I should suppose.
Soc. Then, my friend, I remark with surprise that you have not answered the question which I asked. For I certainly did not ask you to tell me what action is both pious and impious: but now it would seem that what is loved by the gods is also hated by them. [...] How do I know anything more of the nature of piety and impiety? for granting that this action may be hateful to the gods, still piety and impiety are not adequately defined by these distinctions, for that which is hateful to the gods has been shown to be also pleasing and dear to them." And therefore, Euthyphro, I do not ask you to prove this; [...] but I will amend the definition so far as to say that what all the gods hate is impious, and what they love pious or holy; and what some of them love and others hate is both or neither. Shall this be our definition of piety and impiety?
Euth. We should enquire; and I believe that the statement will stand the test of enquiry.
Soc. We shall know better, my good friend, in a little while. The point which I should first wish to understand is whether the pious or holy is beloved by the gods because it is holy, or holy because it is beloved of the gods.
Euth. I do not understand your meaning, Socrates.
Soc. I will endeavour to explain: we, speak of carrying and we speak of being carried, of leading and being led, seeing and being seen. You know that in all such cases there is a difference, and you know also in what the difference lies? [...] My meaning is, that any state of action or passion implies previous action or passion. It does not become because it is becoming, but it is in a state of becoming because it becomes; neither does it suffer because it is in a state of suffering, but it is in a state of suffering because it suffers. Do you not agree?
Euth. Yes.
Soc. And what do you say of piety, Euthyphro: is not piety, according to your definition, loved by all the gods?
Euth. Yes.
Soc. Because it is pious or holy, or for some other reason?
Euth. No, that is the reason.
Soc. It is loved because it is holy, not holy because it is loved?
Euth. Yes.
Soc. And that which is dear to the gods is loved by them, and is in a state to be loved of them because it is loved of them?
Euth. Certainly.
Soc. Then that which is dear to the gods, Euthyphro, is not holy, nor is that which is holy loved of God, as you affirm; but they are two different things.
Euth. How do you mean, Socrates?
Soc. I mean to say that the holy has been acknowledge by us to be loved of God because it is holy, not to be holy because it is loved.
Euth. Yes.
Soc. But that which is dear to the gods is dear to them because it is loved by them, not loved by them because it is dear to them.
Euth. True.
Soc. But, friend Euthyphro, if that which is holy is the same with that which is dear to God, and is loved because it is holy, then that which is dear to God would have been loved as being dear to God; but if that which dear to God is dear to him because loved by him, then that which is holy would have been holy because loved by him. But now you see that the reverse is the case, and that they are quite different from one another. For one (theophiles) is of a kind to be loved cause it is loved, and the other (osion) is loved because it is of a kind to be loved. [...] And does piety or holiness, which has been defined to be the art of attending to the gods, benefit or improve them? Would you say that when you do a holy act you make any of the gods better?
Euth. No, no; that was certainly not what I meant.
Soc. And I, Euthyphro, never supposed that you did. I asked you the question about the nature of the attention, because I thought that you did not.
Euth. You do me justice, Socrates; that is not the sort of attention which I mean.
Soc. Good: but I must still ask what is this attention to the gods which is called piety?
Euth. It is such, Socrates, as servants show to their masters.
Soc. I understand-a sort of ministration to the gods.
Euth. Exactly. [...] I have told you already, Socrates, that to learn all these things accurately will be very tiresome. Let me simply say that piety or holiness is learning, how to please the gods in word and deed, by prayers and sacrifices. Such piety, is the salvation of families and states, just as the impious, which is unpleasing to the gods, is their ruin and destruction.