The characteristic appellation of a divine spirit in the oldest stratum of the Roman religion is not deus, a god, but rather numen, a power: he becomes deus when he obtains a name, and so is on the way to acquiring a definite personality, but in origin he is simply the 'spirit' of the 'animistic' period, and retains something of the spirit's characteristics. Thus among the divinities of the household we shall see later that the Genius and even the Lar Familiaris, though they attained great dignity of conception, and were the centre of the family life, and to some extent of the family morality, never quite rose to the position of full-grown gods; while among the spirits of the field the wildness and impishness of character associated with Faunus and his companion Inuus—almost the cobolds or hobgoblins of the flocks—reflects clearly the old 'animistic' belief in the natural evilness of the spirits and their hostility to men. The notion of the numen is always vague and indefinite: even its sex may be uncertain. 'Be thou god or goddess' is the form of address in the farmer's prayer already quoted from Cato: 'be it male or female' is the constant formula in liturgies and even dedicatory inscriptions of a much later period.
These spirits are, as we have seen, indwellers in the objects of nature and controllers of the phenomena of nature: but to the Roman they were more. Not merely did they inhabit places and things, but they presided over each phase of natural development, each state or action in the life of man. Varro, for instance, gives us a list of the deities concerned in the early life of the child, which, though it bears the marks of priestly elaboration, may yet be taken as typical of the feeling of the normal Roman family. There is Vaticanus, who opens the child's mouth to cry, Cunina, who guards his cradle, Edulia and Potina, who teach him to eat and drink, Statilinus, who helps him to stand up, Adeona and Abeona, who watch over his first footstep, and many others each with his special province of protection or assistance. The farmer similarly is in the hands of a whole host of divinities who assist him at each stage of ploughing, hoeing, sowing, reaping, and so forth.
If the numen then lacks personal individuality, he has a very distinct specialisation of function, and if man's appeal to the divinity is to be successful, he must be very careful to make it in the right quarter: it was a stock joke in Roman comedy to make a character 'ask for water from Liber, or wine from the nymphs.' Hence we find in the prayer formulæ in Cato and elsewhere the most careful precautions to prevent the accidental omission of the deity concerned: usually the worshipper will go through the whole list of the gods who may be thought to have power in the special circumstances; sometimes he will conclude his prayer with the formula 'whosoever thou art,' or 'and any other name by which thou mayest desire to be called.' The numen is thus vague in his conception but specialised in his function, and so later on, when certain deities have acquired definite names and become prominent above the rest, the worshipper in appealing to them will add a cult-title, to indicate the special character in which he wishes the deity to hear: the woman in childbirth will appeal to Iuno Lucina, the general praying for victory to Iuppiter Victor, the man who is taking an oath to Iuppiter as the deus Fidius. As a still later development the cult-title will, as it were, break off and set up for itself, usually in the form of an abstract personification: Iuppiter, in the two special capacities just noted, gives birth to Victoria and Fides.
The conception of the numen being so formless and indefinite, it is not surprising that in the genuine Roman religion there should have been no anthropomorphic representations of the divinity at all. 'For 170 years,' Varro tells us, taking his date from the traditional foundation of the city in 754 B.C., 'the Romans worshipped their gods without images,' and he adds the characteristic comment, 'those who introduced representations among the nations, took away fear and brought in falsehood.' Symbols of a few deities were no doubt recognised: we have noticed already the silex of Iuppiter and the boundary-stone of Terminus, which were probably at an earlier period themselves objects of worship, and to these we may add the sacred spears of Mars, and the sigilla of the State-Penates. But for the most part the numina were without even such symbolic representation, nor till about the end of the regal period was any form of temple built for them to dwell in. The sacred fire of Vesta near the Forum was, it is true, from the earliest times enclosed in a building; this, however, was no temple, but merely an erection with the essentially practical purpose of preventing the extinction of the fire by rain. The first temple in the full sense of the word was according to tradition built by Servius Tullius to Diana on the Aventine: the tradition is significant, for Diana was not one of the di indigetes, the old deities of the 'Religion of Numa,' but was introduced from the neighbouring town of Aricia, and the attribution to Servius Tullius nearly always denotes an Etruscan or at any rate a non-Roman origin. There were, however, altars in special places to particular deities, built sometimes of stone, sometimes in a more homely manner of earth or sods. We hear for instance of the altar of Mars in the Campus Martius, of Quirinus on the Quirinal, of Saturnus at the foot of the Capitol, and notably of the curious underground altar of Consus on what was later the site of the Circus Maximus. But more characteristic than the erection of altars is the connection of deities with special localities. Naturally enough in the worship of the household Vesta had her seat at the hearth, Ianus at the door, and the 'gods of the storehouse' (Penates) at the cupboard by the hearth, but the same idea appears too in the state-cult. Hilltops, groves, and especially clearings in groves (luci) are the most usual sacred localities. Thus Quirinus has his own sacred hill, Iuppiter is worshipped on the Capitol, Vesta and Iuno Lucina have their sacred groves within the boundaries of the city, and Dea Dia, Robigus, and Furrina similar groves at the limits of Roman territory. The record of almost every Roman cult reveals the importance of locality in connection with the di indigetes, and the localities are usually such as would be naturally chosen by a pastoral and agricultural people. http://ancient-roman-religion.blogspot.com/2009/01/roman-theology.html