That God is Not Corruptible, Who, If He Were, Would Not Be God at All.

6. For I was so struggling to find out the rest, as having already found that what was incorruptible must be better than the corruptible; and Thee, therefore, whatsoever Thou wert, did I acknowledge to be incorruptible. For never yet was, nor will be, a soul able to conceive of anything better than Thou, who art the highest and best good. But whereas most truly and certainly that which is incorruptible is to be preferred to the corruptible (like as I myself did now prefer it), then, if Thou were not incorruptible, I could in my thoughts have reached unto something better than my God. Where, then, I saw that the incorruptible was to be preferred to the corruptible, there ought I to seek Thee, and there observe “whence evil itself was,” that is, whence comes the corruption by which Thy substance can by no means be profaned. For corruption, truly, in no way injures our God,—by no will, by no necessity, by no unforeseen chance,—because He is God, and what He wills is good, and Himself is that good; but to be corrupted is not good. Nor art Thou compelled to do anything against Thy will in that Thy will is not greater than Thy power. But greater should it be wert Thou Thyself greater than Thyself; for the will and power of God is God Himself. And what can be unforeseen by Thee, who knowest all things? Nor is there any sort of nature but Thou knowest it. And what more should we say “why that substance which God is should not be corruptible,” seeing that if it were so it could not be God?

Conf 9.5.1