I. Introductory.
Leo, bishop, to Turribius, bishop, greeting.
Your laudable zeal for the truth of the Catholic Faith, and the painstaking devotion you expend in the exercise of your pastoral office upon the Lord's flock is proved by your letter, brother, which your deacon has handed to us, in which you have taken care to bring to our knowledge the nature of the disease which has burst forth in your district from the remnants of an ancient plague. For the language of your letter, and your detailed statement, and the text of your pamphlet, explains clearly that the filthy puddle of the Priscillianists again teems with life among you. For there is no dirt which has not flowed into this dogma from the notions of all sorts of heretics: since they have scraped together the motley dregs from the mire of earthly opinions and made for themselves a mixture which they alone may swallow whole, though others have tasted little portions of it.
In fact, if all the heresies which have arisen before the time of Priscillian were to be studied carefully, hardly any mistake will be discovered with which this impiety has not been infected: for not satisfied with accepting the falsehoods of those who have departed from the Gospel under the name of Christ, it has plunged itself also in the shades of heathendom, so as to rest their religious faith and their moral conduct upon the power of demons and the influences of the stars through the blasphemous secrets of the magic arts and the empty lies of astrologers. But if this may be believed and taught, no reward will be due for virtues, no punishment for faults, and all the injunctions not only of human laws but also of the Divine constitutions will be broken down: because there will be no criterion of good or bad actions possible, if a fatal necessity drives the impulses of the mind to either side, and all that men do is through the agency not of men but of stars. To this madness belongs that monstrous division of the whole human body among the twelve signs of the zodiac, so that each part is ruled by a different power: and the creature, whom God made in His own image, is as much under the domination of the stars as his limbs are connected one with the other. Rightly then our fathers, in whose times this abominable heresy sprung up, promptly pursued it throughout the world, that the blasphemous error might everywhere be driven from the Church: for even the leaders of the world so abhorred this profane folly that they laid low its originator, with most of his disciples, by the sword of the public laws. For they saw that all desire for honourable conduct was removed, all marriage-ties undone, and the Divine and the human law simultaneously undermined, if it were allowed for men of this kind to live anywhere under such a creed. And this rigourous treatment was for long a help to the Church's law of gentleness which, although it relies upon the priestly judgment, and shuns blood-stained vengeance, yet is assisted by the stern decrees of Christian princes at times when men, who dread bodily punishment, have recourse to merely spiritual correction. But since many provinces have been taken up with the invasions of the enemy, the carrying out of the laws also has been suspended by these stormy wars. And since intercourse came to be difficult among God's priests and meetings rare, secret treachery was free to act through the general disorder, and was roused to the upsetting of many minds by those very ills which ought to have counteracted it. But which of the peoples and how many of them are free from the contagion of this plague in a district where, as you point out, dear brother, the minds even of certain priests have sickened of this deadly disease: and they who were believed the necessary quellers of falsehood and champions of the Truth are the very ones through whom the Gospel of God is enthralled to the teaching of Priscillian: so that the fidelity of the holy volumes being distorted to profane meanings, under the names of prophets and apostles, is proclaimed not that which the Holy Spirit has taught, but what the devil's servant has inserted. Therefore as you, beloved, with all the faithful diligence in your power, have dealt under 16 heads with these already condemned opinions, we also subject them once more to a strict examination; lest any of these blasphemies should be thought either bearable or doubtful.
Source: Letters (New Advent)