13.  Heretics have dared to say that there are two Gods, and of good and evil two sources, and these unbegotten.  If both are unbegotten it is certain that they are also equal, and both mighty.  How then doth the light destroy the darkness?  And do they ever exist together, or are they separated?  Together they cannot be; for what fellowship hath light with darkness? saith the Apostle 2 Cor. vi. 14.  Cyril’s description applies especially to the heresy of Manes.  See § 36, note 3, at the end of this Lecture; also Cat. xi. 21. and Cat. xv. 3..  But if they are far from each other, it is certain that they hold also each his own place; and if they hold their own separate places, we are certainly in the realm of one God, and certainly worship one God.  For thus we must conclude, even if we assent to their folly, that we must worship one God.  Let us examine also what they say of the good God.  Hath He power or no power?  If He hath power, how did evil arise against His will?  And how doth the evil substance intrude, if He be not willing?  For if He knows but cannot hinder it, they charge Him with want of power; but if He has the power, yet hinders not, they accuse Him of treachery.  Mark too their want of sense.  At one time they say that the Evil One hath no communion with the good God in the creation of the world; but at another time they say that he hath the fourth part only.  Also they say that the good God is the Father of Christ; but Christ they call this sun.  If, therefore according to them, the world was made by the Evil One, and the sun is in the world, how is the Son of the Good an unwilling slave in the kingdom of the Evil?  We bemire ourselves in speaking of these things, but we do it lest any of those present should from ignorance fall into the mire of the heretics.  I know that I have defiled my own mouth and the ears of my listeners:  yet it is expedient.  For it is much better to hear absurdities charged against others, than to fall into them from ignorance:  far better that thou know the mire and hate it, than unawares fall into it.  For the godless system of the heresies is a road with many branches, and whenever a man has strayed from the one straight way, then he falls down precipices again and again.