At the head of the whole system was Jupiter, the greatest of Roman gods, whose title of Optimus Maximus might well have suggested that no other deity could occupy this place. Without him it would have been practically impossible for Varro to carry out his difficult and perilous task. Every Roman recognised in Jupiter the god who condescended to dwell on the Capitol in a temple made with hands, and who, beyond all other gods, watched over the destinies of the Roman State; every Roman also knew that Jupiter was the great god of the heaven above him, for in many expressions of his ordinary speech he used the god's name as a synonym for the open sky.[556] The position now accorded to the heaven-god in the new Stoic system is so curious and interesting that we must dwell on it for a moment.
Varro held, or at any rate taught, that Jupiter was himself that soul of the world (animus mundi) which fills and moves the whole material universe.[557] He is the one universal causal agent,[558] from whom all the forces of nature are derived;[559] or he may be called, in language which would be intelligible to the ordinary Roman, the universal Genius.[560] Further, he is himself all the other gods and goddesses, who may be described as parts, or powers, or virtues, existing in him.[561] And Varro makes it plain that he wishes to identify this great god of gods with the Jupiter at Rome, whose temple was on the Capitol; St. Augustine quotes him as holding that the Romans had dedicated the Capitol to Jupiter, who by his spirit breathes life into everything in the universe:[562] or in less philosophical language, "The Romans wish to recognise Jupiter as king of gods and men, and this is shown by his sceptre and his seat on the Capitol." Thus the god who dwelt on the Capitol, and in the temple which was the centre-point of the Roman Empire, was also the life-giving ruler and centre of the whole universe. Nay, he goes one step further, and identifies him with the one God of the monotheistic peoples of the East, and in particular with the God of the Jews.[563]
Thus Varro had arrived, with the help of Posidonius and the Stoics, at a monotheistic view of the Deity, which is at the same time a kind of pantheism, and yet, strange to say, is able to accommodate itself to the polytheism of the Graeco-Roman world. But without Jupiter, god of the heaven both for Greeks and Romans, and now too in the eyes of both peoples the god who watched over the destiny of the Roman Empire, this wonderful feat could not have been performed. The identification of the heaven-god with the animus mundi of the Stoics was not indeed a new idea; it may be traced up Stoic channels even to Plato. What is really new and astonishing is that it should have been possible for a conservative Roman like Varro, in that age of carelessness and doubt, to bring the heaven-god, so to speak, down to the Roman Capitol, where his statue was to be seen sitting between Juno and Minerva, and yet to teach the doctrine that he was the same deity as the Jewish Jehovah, and that both were identical with the Stoic animus mundi.