Ambrose, Bishop of Milan, responded to this request with two letters to Valentinian, at that point emperor in the West. These letters, Ep. 17 and Ep. 18, may be found elsewhere on this site.
Ambrose, Ep. 17 & 18
The translation is that in the Library of Nicene and Post Nicene Fathers and available in its entirety on the World Wide Web, together with the other volumes in the series. I have replaced the introduction with a brief one of my own, removed the page numbers to that edition (since they seemed more confusing than helpful), and deleted the references to footnotes (since only the references and not the notes were present in the scanned version). I have added a few notes; these, employing the capabilities of Internet browsers, I have placed in a second frame to facilitate consultation of text and notes at the same time. Occasionally, I have made a few corrections to the scanned text and made minor changes to layout to facilitate ease of use.
Symmachus, Relation 3 
I. As soon as the most honourable Senate, always devoted to you, knew that crimes were made amenable to law, and that the reputation of late times was being purified by pious princes, it, following the example of a more favourable time, gave utterance to its long suppressed grief, and bade me be once again the delegate to utter its complaints. But through wicked men audience as refused me by the divine Emperor, otherwise justice would not have been wanting, my lords and emperors, of great renown, Valentinian, Theodosius, and Arcadius, victorious and triumphant, ever august.
2. In the exercise, therefore, of a twofold office, as your Prefect I attend to public business, and as delegate I recommend to your notice the charge laid on me by the citizens. Here is no disagreement of wills, for men have now ceased to believe that they excel in courtly zeal, if they disagree. To be loved, to be reverenced, to be esteemed is more than imperial sway. Who could endure that private disagreement should injure the state? Rightly does the Senate censure those who have preferred their own power to the reputation of the prince.
3. But it is our task to watch on behalf of your Graces. For to what is it more suitable that we defend the institutions of our ancestors, and the rights and destiny of our country, than to the glory of these times, which is all the greater when you understand that you may not do anything contrary to the custom of your ancestors? We demand then the restoration of that condition of religious affairs which was so long advantageous to the state. Let the rulers of each sect and of each opinion be counted up; a late one practised the ceremonies of his ancestors, a later did not put them away. If the religion of old times does not make a precedent, let the connivance of the last do so. 
4. Who is so friendly with the barbarians as not to require an Altar of Victory?  We will be careful henceforth, and avoid a show of such things. But at least let that honour be paid to the name which is refused to the goddess--your fame, which will last for ever, owes much and will owe still more to victory. Let those be averse to this power, whom it has never benefited. Do you refuse to desert a patronage which is friendly to your triumphs? That power is wished for by all, let no one deny that what he acknowledges is to be desired should also be venerated.
5. But even if the avoidance of such an omen were not sufficient, it would at least have been seemly to abstain from injuring the ornaments of the Senate House. Allow us, we beseech you, as old men to leave to posterity what we received as boys. The love of custom is great. Justly did the act of the divine Constantius last but for a short time.  All precedents ought to be avoided by you, which you know were soon abolished. We are anxious for the permanence of your glory and your name, that the time to come may find nothing which needs correction.
6. Where shall we swear to obey your laws and commands? By what religious sanction shall the false mind be terrified, so as not to lie in bearing witness? All things are indeed filled with God, and no place is safe for the perjured, but to be urged in the very presence of religious forms has great power in producing a fear of sinning. That altar preserves the concord of all, that altar appeals to the good faith of each, and nothing gives more authority to our decrees than that the whole of our order issues every decree as it were under the sanction of an oath. So that a place will be opened to perjury, and this will be determined by my illustrious Princes, whose honour is defended by a public oath.
7. But the divine Constantius is said to have done the same. Let us rather imitate the other actions of that Prince, who would have undertaken nothing of the kind, if any one else had committed such an error before him. For the fall of the earlier sets his successor right, and amendment results from the censure of a previous example. It was pardonable for your Grace's ancestor in so novel a matter to fail in guarding against blame. Can the same excuse avail us if we imitate what we know to have been disapproved?
8. Will your Majesties listen to other actions of this same Prince, which you may more worthily imitate? He diminished none of the privileges of the sacred virgins, he filled the priestly offices with nobles, he did not refuse the cost of the Roman ceremonies, and following the rejoicing Senate through all the streets of the eternal city, he contentedly beheld the shrines with unmoved countenance, he read the names of the gods inscribed on the pediments, he enquired about the origin of the temples, and expressed admiration for their builders.  Although he himself followed another religion, he maintained its own for the empire, for everyone has his own customs, everyone his own rites. The divine Mind has distributed different guardians and different cults to different cities.  As souls are separately given to infants as they are born, so to peoples the genius of their destiny. Here comes in the proof from advantage, which most of all vouches to man for the gods. For, since our reason is wholly clouded, whence does the knowledge of the gods more rightly come to us, than from the memory and evidence of prosperity? Now if a long period gives authority to religious customs, we ought to keep faith with so many centuries, and to follow our ancestors, as they happily followed theirs.
9. Let us now suppose that Rome is present and addresses you in these words: "Excellent princes, fathers of your country, respect my years to which pious rites have brought me. Let me use the ancestral ceremonies, for I do not repent of them. Let me live after my own fashion, for I am free. This worship subdued the world to my laws, these sacred rites repelled Hannibal from the walls, and the Senones from the capitol. Have I been reserved for this, that in my old age I should be blamed? I will consider what it is thought should be set in order, but tardy and discreditable is the reformation of old age." 
10. We ask, then, for peace for the gods of our fathers and of our country. It is just that all worship should be considered as one. We look on the same stars, the sky is common, the same world surrounds us. What difference does it make by what pains each seeks the truth? We cannot attain to so great a secret by one road;  but this discussion is rather for persons at ease, we offer now prayers, not conflict.
11. With what advantage to your treasury are the prerogatives of the Vestal Virgins diminished? Is that refused under the most bountiful emperors which the most parsimonious have granted? Their sole honour consists in that, so to call it, wage of chastity. As fillets are the ornament of their heads, so is their distinction drawn from their leisure to attend to the offices of sacrifice. They seek for in a measure the empty name of immunity, since by their poverty they are exempt from payment. And so they who diminish anything of their substance increase their praise, inasmuch as virginity dedicated to the public good increases in merit when it is without reward.
12. Let such gains as these be far from the purity of your treasury. Let the revenue of good princes be increased not by the losses of priests, but by the spoils of enemies. Does any gain compensate for the odium? And because no charge of avarice falls upon your characters, they are the more wretched whose ancient revenues are diminished. For under emperors who abstain from what belongs to others, and resist avarice, that which does not move the desire of him who takes it, is taken solely to injure the loser.
13. The treasury also retains lands bequeathed to virgins and ministers by the will of dying persons.  I entreat you, priests of justice, let the lost right of succession be restored to the sacred persons and places of your city. Let men dictate their wills without anxiety, and know that what has been written will be undisturbed under princes who are not avaricious. Let the happiness in this point of all men give pleasure to you, for precedents in this matter have begun to trouble the dying. Does not then the religion of Rome appertain to Roman law? What name shall be given to the taking away of property which no law nor accident has made to fail? Freedmen take legacies, slaves are not denied the just privilege of making wills; only noble virgins and the ministers of sacred rites are excluded from property sought by inheritance. What does it profit the public safety to dedicate the body to chastity, and to support the duration of the empire with heavenly guardianship, to attach the friendly powers to your arms and to your eagles, to take upon oneself vows efficacious for all, and not to have common rights with all? So, then, slavery is a better condition, which is a service rendered to men. We injure the State, whose interest it never is to be ungrateful.
14. And let no one think that I am defending the cause of religion only. For from deeds of this kind have arisen all the misfortunes of the Roman race. The law of our ancestors honoured the Vestal Virgins and the ministers of the gods with a moderate maintenance and just privileges. This grant remained unassailed till the time of the degenerate money-changers, who turned the fund for the support of sacred chastity into hire for common porters. A general famine followed upon this, and a poor harvest disappointed the hopes of all the provinces. This was not the fault of the earth, we impute no evil influence to the stars. Mildew did not injure the crops, nor wild oats destroy the corn; the year failed through the sacrilege, for it was necessary that what was refused to religion should be denied to all. 
15. Certainly, if there be any instance of this evil, let us impute such a famine to the power of the season. A deadly wind has been the cause of this barrenness, life is sustained by trees and shrubs, and the need of the country folk has betaken itself once more to the oaks of Dodona. What similar evil did the provinces suffer, so long as the public charge sustained the ministers of religion? When were the oaks shaken for the use of men, when were the roots of plants torn up, when did fertility on all sides forsake the various lands, when supplies were in common for the people and for the sacred virgins? For the support of the priests was a blessing to the produce of the earth, and was rather an insurance than a bounty. Is there any doubt that what was given was for the benefit of all, seeing that the want of all has made this plain?
16. But some one will say that public support is only refused to the cost of foreign religions. Far be it from good princes to suppose that what has been given to certain persons from the common property can be in the power of the treasury. For as the State consists of individuals, that which goes out from it becomes again the property of individuals. You rule over all; but you preserve his own for each individual; and justice has more weight with you than arbitrary will. Take counsel with your own liberality whether that which you have conferred on others ought to be considered public property. Sums once given to the honour of the city cease to be the property of those who have given them, and that which at the commencement was a gift, by custom and time becomes a debt. Any one is therefore endeavouring to impress upon your minds a vain fear, who asserts that you share the responsibility of the givers unless you incur the odium of withdrawing the girls.
17. May the unseen guardians of all sects be favourable to your Graces, and may they especially, who in old time assisted your ancestors, defend you and be worshipped by us. We ask for that state of religious matters which preserved the empire for the divine parent of your Highnesses, and furnished that blessed prince with lawful heirs. That venerable father beholds from the starry height the tears of the priests, and considers himself censured by the violation of that custom which he willingly observed. 
18. Amend also for your divine brother that which he did by the counsel of others, cover over the deed which he knew not to be displeasing to the Senate.  For it is allowed that legation was denied access to him, lest public opinion should reach him. It is for the credit of former times, that you should not hesitate to abolish that which is proved not to have been the doing of the prince.
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