Plato, in his construction of the ideal state, made "impiety" a crime punishable by five years imprisonment for the first offense and death upon a second conviction.
[Smith, p 4; Plato, Laws, trans. Jowett, 1952, vol. 7, pp. 769-770]

Athenian: Every means, then, shall we say, must be employed to keep our children from the desire to reproduce different models in dance or song, as well as to prevent a possible tempter from offering them the inducement of a variety of delights?
Clinias: Perfectly true.

Athenian: Well, can any of us find a better device for this purpose than that employed in Egypt?
Clinias: And what is that?

Athenian: Why, the plan is to consecrate all our dances and all our tunes. First, the festivals must be fixed by compiling an annual calendar to show what feasts are to be celebrated, at what dates, and in honor of what deities, sons of deities, or spirits, respectively. Next, certain authorities must determine what hymn is to be sung on the feast of each divinity, and by what dances the ceremony of the day is to be graced. When this has been determined, the whole citizen body must do public sacrifice to the Destinies and the entire pantheon at large, and consecrate each hymn to its respective god or other patron by solemn libation. If any man tries to introduce hymn or dance into the worship of any deity in contravention of these canons, the priests of either sex, acting in conjunction with the curators of law, shall have the warrant both of religion and law in excluding him from the festival; if the excluded party declines to submit to this excommunication, he shall for life be liable to indictment for impiety at the instance of any who cares to institute proceedings.
Laws VII: 798e-799b, p 1370-1

Athenian: Well, then, let us, I say, take the paradox as granted. Our songs have become canons, ... In fine, let us assume a clause on the subject to the following effect. No man shall contravene the public standards of song, ritual, or choric performance of the young at large, whether by vocal utterance or by movement in the dance, any more than he would any other of our canons. Conformity shall be clear of the law; nonconformity shall be visited with penalties by curators of law and priests of either sex as before enjoined.
Laws VII: 799e-800b, p 1371

Athenian: No man who believes in gods as the law would have him believe has ever yet of his own free will done unhallowed deed or let slip lawless discourse. If a man acts thus, 'tis from one of three causes. Either, as I say, he does not believe, or again, he believes that they are, but are regardless of mankind, or lastly, that they are lightly to be won over by the cajoling of offerings and prayers.
Laws X:885b, p 1440-1

Athenian: You imagine that what impels their souls to irreligion is incontinence of pleasures and lusts, and nothing more.
Laws X:886b, p 1441

Athenian: Must we look on ourselves as, so to say, indicted at the bar of the ungodly and defend our incriminated legislation from the charge that it has no right to assume the existence of gods?
Laws X:886e-887a, p 1442

Athenian: ... how, I ask, is a man to find gentle language in which to combine reproof with instruction in the initial truth about the gods--that of their existence? Still, the task is to be faced. We can never permit one party among us to run mad from lust of pleasure, and the rest equally mad from fury against them.
Laws X:888a, p 1443

Athenian: ... Hence I, who have had the acquaintance of many such, can assure you that no one who in early life has adopted this doctrine of the nonexistence of gods has ever persisted to old age constant to that conviction, though there have been cases--not many, certainly, but still some few--of persistence in the other two attitudes, the belief that there are gods but that they are indifferent to human conduct, and again, that, though not indifferent, they are lightly placated by sacrifice and prayers. If you will be ruled by me, then, you will wait for the fullness of clear and confident judgment on these matters to come to you, and inquire whether truth lies in one direction or another, seeking for guidance in all quarters, and above all from the legislator. Meanwhile, beware of all impiety toward the gods. For he who is framing the law for you must make it his business, hereafter as well as now, to instruct you in the truth of this matter.
Laws X:888b-d, p 1444

Athenian: ... Statesmanship in especial, they say, is a thing which has a little in common with nature, but is mainly a business of art; legislation, likewise, is altogether an affair not of nature, but of art, and its positions are unreal.
Clinias: Unreal--but how so?

Athenian: Why, my dear sir, to begin with, this party asserts that gods have no real and natural, but only an artificial being, in virtue of legal conventions, as they call them, and thus there are different gods for different places, conformably to the convention made by each group among themselves when they drew up their legislation. Then they actually declare that the really and naturally laudable is one thing and the conventionally laudable quite another, while as for right, there is absolutely no such thing as a real and natural right, ...
Hence our epidemics of youthful irreligions--as though there were no gods such as the law enjoins us to believe in--and hence the factions created by those who seek, on such grounds, to attract men to the 'really and naturally right life,' that is, the life of real domination over others, not of conventional service to them.
Clinias: What an awful creed you describe, sir! What a general corruption of the young people of whole cities and private households!

Athenian: Too true, Clinias, too true. But how would you have the legislator act where such a situation is of long standing? Should he be content to stand up in public and threaten people all round that unless they confess the being of gods, and believe in their hearts that they are such as his law declares--and the case is the same with the laudable, the right, and everything of highest moment, and all that makes for virtue or vice--action must conform in all cases to the convictions prescribed by the text of the legislation-is he to threaten, I say, that those who will not lend a ready ear to the laws shall in some cases suffer death, in others be visited with bonds and whipping, in others with infamy, and in yet others with poverty and banishment, but to have no words of persuasion with which to work on his people, as he dictates their laws, and so, it may be, tame them?
Clinias: Far from it, sir, far from it. If there are indeed persuasives, however weak, in such matters, no legislator who deserves the slightest consideration must ever faint. He should strain every nerve, as they say, to plead in support of the old traditional belief of the being of gods and of all you have just recounted. In especial also, he should defend the claim of law itself and of art to be natural, or no less real than nature, seeing that they are products of mind ...
Laws X:889d-890d, p 1445-6

Athenian: So, that your present creed may lead you to no worse pitch of impiety, that the specter, as we may say, may happily be laid, as it approaches, by the power of argument, we must try to connect what now remains to be said with our original rejoinder to the complete atheist, and so have the benefit of that also.
Laws X: 900b, p 1456

Athenian: Now are not the gods, one and all, our chiefest guardians, and the interests they guard our chief interests?
Clinias: Aye, and by far. ... Of all reprobates who are given to any form of ungodliness the defender of such a creed may well be most righteously condemned as the very worst and most ungodly.

Athenian: Then I presume we may say our three propositions, that there are gods, that they are mindful of us, that they are never to be seduced from the path of right, are sufficiently demonstrated. ...
So our preamble may properly be followed by a sentence which will express the sense of our laws, a general injunction to the ungodly to turn from their ways to those of godliness. For the disobedient our law against impiety may run as follows. If any man commit impiety of word or act, any person present shall defend the law by giving information to the magistrates, and the first magistrates under whose notice the matter comes shall bring the case before the court appointed to deal with such offenses as the law directs. Any official failing to take action on information received shall himself be liable to be proceeded against for impiety at the suit of anyone willing to vindicate the law. In the case of conviction, the court shall impose a particular penalty on the offender for each act of impiety. Imprisonment shall form part of the penalty in all cases. And whereas there are three prisons in the state, a common jail in the market place for the majority of cases, for safe custody of the persons of the commonalty, a second attached to the nocturnal council and known as the house of correction, and a third in the heart of the country in the most solitary and wildest situation available, and called by some designation suggestive of punishment, and whereas also there are three causes of impiety, those we have already specified, and each such cause gives rise to two types of offense, there will be, in all, six classes of offenders against religion to be discriminated, who require different and dissimilar treatment. For though a man should be a complete unbeliever in the being of gods, if he have also a native uprightness of temper, he will detest evil men, his repugnance to wrong disinclines him to commit wrongful acts, he shuns the unrighteous and is drawn to the upright. But those in whom the conviction that the world has no place in it for gods is conjoined with incontinence of pleasure and pain and the possession of a vigorous memory and a keen intelligence share the malady of atheism with the other sort, but are sure to work more harm, where the former do less, in the way of mischief to their fellows. The first man may probably be free-spoken enough about gods, sacrifices, and oaths, and perhaps, if he does not meet with his deserts, his mockery may make converts of others. But the second, who holds the same creed as the other, but is what is popularly called a 'man of parts,' a fellow of plentiful subtlety and guile-that is the type which furnishes our swarms of diviners and fanatics for all kinds of imposture; on occasion also it produces dictators, demagogues, generals, contrivers of private Mysteries, and the arts and tricks of the so-called Sophist. Thus there are numerous types of these atheists, but two which legislation must take into account, the hypocritical, whose crimes deserve more than one death, or even two, and the others, who call for the combination of admonition with confinement. Similarly, the belief in divine indifference gives rise to two further types, and that in divine venality to another two.
These distinctions once recognized, the law shall direct the judge to commit those whose fault is due to folly apart from viciousness of temper or disposition to the house of correction for a term of not less than five years. Throughout this period they shall have no communication with any citizen except the members of the nocturnal council, who shall visit them with a view to admonition and their soul's salvation. When the term of confinement has expired, if the prisoner is deemed to have returned to his right mind, he shall dwell with the right-minded, but if not, and he be condemned a second time on the same charge, he shall suffer the penalty of death. As for those who add the character of a beast of prey to their atheism or belief in divine indifference or venality, those who in their contempt of mankind bewitch so many of the living by the pretense of evoking the dead and the promise of winning over the gods by the supposed sorceries of prayer, sacrifice, and incantations, and thus do their best for lucre to ruin individuals, whole families, and communities, the law shall direct the court to sentence a culprit convicted of belonging to this class to incarceration in the central prison, where no free citizen whatsoever shall have access to him, and where he shall receive from the turnkeys the strict rations prescribed by the curators of the laws. At death he shall be cast out beyond the borders without burial, and if any free citizen has a hand in his burial, he shall be liable to a prosecution for impiety at the suit of any who cares to take proceedings. ...
Moreover we must frame a law applicable to all these offenders alike, and designed to alleviate the sin of most of them against religion in word or act-to say nothing of the folly of the sinners-by the prohibition of illegal ceremonial. In fact the following law should be enacted for all cases without exception. No man shall possess a shrine in his private house; when a man feels himself moved to offer sacrifice, he shall go to the public temples for that purpose and deliver his offerings to the priests of either sex whose business it is to consecrate them. ...
[no citizen to] worship at any shrine other than the public, ...
Any person proved guilty of a sin against piety which is the crime of a grown man, not the trivial offense of a child, whether by dedicating a shrine on private ground or by doing sacrifice to any gods whatsoever in public, shall suffer death for doing sacrifice in a state of defilement.
Laws XI: 907a-910d, p 1462-5

Athenian: Now among these matters of high import is not the subject of divinity which we treated so earnestly pre-eminent? 'Tis of supreme moment for us, is it not, to know with all the certainty permitted to man that there are gods, and with what evident might they are invested? In the great mass of our citizens we may tolerate mere conformity to the tradition embodied in the laws, but we shall do well to deny all access to the body of our guardians to any man who has not made it his serious business to master every proof there is of the being of gods. And by denial of access I mean that no man who is not divinely gifted or has not labored at divinity shall ever be chosen for a curator, nor ever be numbered among those who win the distinction for virtue.
Laws XII: 966c-d, p 1511

Plato, Laws; Plato: the collected dialogues,
edited by Edith Hamilton and Huntington Cairns (Princeton 1961) 10: 886-99b; p 1440-