In historical times the two kinds of ius, divinum and humanum, were strongly distinguished (see Wissowa, R.K. p. 318, who quotes Gaius ii. 2: "summa itaque rerum divisio in duos articulos diducitur, nam aliae sunt divini iuris, aliae humani"). But it is almost certain that there was originally no such clear distinction. The general opinion of historians of Roman law is thus expressed by Cuq (Institutions juridiques des Romains, p. 54): "Le droit civil n'a eu d'abord qu'une portée fort restreinte. Peu à peu il a gagné du terrain, il a entrepris de réglementer des rapports qui autrefois étaient du domaine de la religion. Pendant longtemps à Rome le droit théocratique a coexisté avec le droit civil." (See also Muirhead, Introduction to Roman Law, ed. Goudy, p. 15.) Possibly the formation of an organised calendar, marking off the days belonging to the deities from those which were not so made over to them, first gave the opportunity for the gradual realisation of the thought that the set of rules under which the citizen was responsible to the divine beings was not exactly the same as that under which he was responsible to the civil authorities. The distinction took many ages to realise in all its aspects, and is not complete even under the XII. Tables or later, because the sanction for civil offences remained in great part a divine one; on this point Jhering is certainly wrong (Geist des röm. Rechts, i. 267 foll.). As Cuq remarks (p. 54, note 1), one institution of the ius divinum kept its force after the complete secularisation of law, and retains it to this day, viz. the oath.
If there was originally no distinction between religious and civil rules of law, it follows that there were originally no two distinguishing terms for them. The earliest passage in which they are distinguished as ius divinum and humanum (so far as I know) is Cicero's speech for Sestius (B.C. 56), sec. 91, quoted by Wissowa, p. 319: "domicilia coniuncta quas urbes dicimus, invento et divino iure et humano, moenibus cinxerunt." But by all British writers on Roman law, and by many foreign ones, the word fas is used as equivalent to the ius divinum, and sharply distinguished from ius. Thus the late Dr. Greenidge, in his useful work on Roman public life (p. 52 and elsewhere), makes this distinction; he writes of the rex as the chief expounder of the divine law (fas), and of the control exercised by fas over the citizen's life. Cp. Muirhead, ed. Goudy, p. 15 foll., where Mommsen is quoted thus: "Mommsen is probably near the mark when he describes the leges regiae as mostly rules of the fas." But Mommsen, like Wissowa in his Religion und Kultus, does not use the word fas, but speaks of "Sakralrecht." Sohm, on the other hand (Roman Law, trans. Ledlie, p. 15, note), compares fas with Sanscrit dharma and Greek themis, as meaning unwritten rules of divine origin, which eventually gave way before ius, as in Greece before [Greek: dikaion]. (Cp. Binder, Die Plebs, p. 501.) But it is safer in this case to leave etymology alone, and to try to discover what the Romans themselves understood by fas, which is indeed a peculiar and puzzling word. (For its possible connection with fari, effari (ager effatus), fanum, and profanum, etc., see H. Nettleship's Contributions to Latin Lexicography, s.v. "Fas.")
Fas was at all times indeclinable, and is rarely found even as an accusative, as in Virg. Aen. ix. 96:
mortaline manu factae immortale carinae fas habeant?
In the oldest examples of its use, i.e. in the ancient calendar QRCF, on March 24 and May 24, i.e. "quando rex comitiavit fas" (Varro, L.L. vi. 31), and QStDF on June 15, i.e. "Quando stercus delatum fas" (Varro, L.L. vi. 32), it is hard to say whether it is a substantive at all, and not rather an adverb like satis. So, too, in the antique language of the lex templi of Furfo (58 B.C.) we read, "Utii tangere sarcire tegere devehere defigere mandare ferro oeti promovere referre fasque esto" (liceat should probably be inserted before fasque esto). See CIL. i. 603, line 7; Dessau, Inscript. Lat. selectae, ii. 1. 4906, p. 246. In these examples fas simply means that you may do certain acts without breaking religious law; it does not stand for the religious law itself. To me it looks like a technical word of the ius divinum, meaning that which it is lawful to do under it; thus a dies fastus is one on which it is lawful under that ius to perform certain acts of civil government, "sine piaculo" (Varro, L.L. vi. 29). Nefas is, therefore, in the same way a word which conveys a prohibition under the divine law. By constant juxtaposition with ius, fas came in course of time to take on the character of a substantive, and so too did its opposite nefas. The dictionaries supply many examples of its use as a substantive and as paralleled with ius, but the only one I can find that is earlier than Cicero is Terence, Hecyra, iii. 3. 27, i.e. in the work of a non-Roman.
I cannot find that it is so used by Varro, where we might naturally have expected it. Cicero does not call his imaginary ius divinum a fas, but iura religionum, constitutio religionum (de Legibus ii. 10-23, 17-32). Ius is the word always used technically of particular departments of the religious law, e.g. ius pontificium, ius augurale, and ius fetiale (CIL. i. p. 202, is preimus ius fetiale paravit). The notion that fas could mean a kind of code of religious law is probably due to Virgil's use of the word in "Quippe etiam festis quaeddam exercere diebus Fas et iura sinunt," Georg. i. 269, and to the comment of Servius, "id est, divina humanaque iura permittunt: nam ad religionem fas, ad homines iura pertinent."
It is strange to find it personified as a kind of deity in the formula of the fetiales, used when they announced the Roman demands at an enemy's frontier (Livy i. 32): "Audi Iuppiter, inquit, audite Fines (cuiuscunque gentis sunt nominat), audiat Fas." Whence did Livy get this formula? We have no record of a book of the fetiales; if this came from those of the pontifices, as is probable, the formula need not be of ancient date, and the personification of Fines also suggests a doubt as to the genuineness of the whole formula.