9 Now that all Faustus' calumnies have been refuted, those at least on the subjects here treated of at large and explained fully as the Lord has enabled me, I close with a word of counsel to you who are implicated in those shocking and damnable errors, that, if you acknowledge the supreme authority of Scripture, you should recognise that authority which from the time of Christ Himself, through the ministry of His apostles, and through a regular succession of bishops in the seats of the apostles, has been preserved to our own day throughout the whole world, with a reputation known to all.
There the Old Testament too has its difficulties solved, and its predictions fulfilled. If you ask for demonstration, consider first what you are, how unfit for comprehending the nature of your own soul, not to speak of God; I mean an intelligent comprehension, such as you profess to desire, or to have once desired, and not the notions of a credulous fancy. Admitting this incompetency, which must continue while you remain as you are, you may at least be referred to the natural conviction of every human mind, unless it is corrupted by error, of the perfect unchangeableness and incorruptibility of the nature and substance of God. Admit this, or believe it, and you will no longer be Manichæans, so that in course of time you may become Catholics.
Source: Reply to Faustus the Manichaean (New Advent)