John Cassian, Conferences I–X VI.16: How even celestial powers above are capable of change

But that even the powers above are, as we said, subject to change is shown by those who fell from their ranks through the fault of a corrupt will. Wherefore we ought not to think that the nature of those is unchangeable, who remain in the blessed condition in which they were created, simply because they were not in like manner led astray to choose the worse part. For it is one thing to have a nature incapable of change, and another thing for a man through the efforts of his virtue, and by guarding what is good through the grace of the unchangeable God, to be kept from change. For everything that is secured or preserved by care, can also be lost by carelessness. And so we read: “Call no man blessed before his death,” because so long as a man is still engaged in the struggle, and if I may use the expression, still wrestling—even though he generally conquers and carries off many prizes of victory,—yet he can never be free from fear, and from the suspicion of an uncertain issue. And therefore God alone is called unchangeable and good, as His goodness is not the result of effort, but a natural possession, and so He cannot be anything but good. No virtue then can be acquired by man without the possibility of change, but in order that when it once exists it may be continually preserved, it must be watched over with the same care and diligence with which it was acquired.