Now against all this, as its chief enemy, though he may not know it, stands the old Catholic philosophy of Man. The first and last idea of it is Resurrection, that is the resurrection of the whole of man. It is, as I have said before, a mystical refusal to despair of the original pre-historic monster of that name. It is true that the older creed often demands amputation in the sense of asceticism, as in the text about cutting off the hand to enter Heaven. But the difference is instantly made vivid by the rest of the text, which declares that even such amputation is better than casting the whole body of man into Hell. But the Shavian evolutionist does really want to cast the whole body of man into Chaos. He wants to cast it into the melting-pot, and boil it to nothing, that a new and superior something may at last emerge. This is indeed much more than amputation, it is annihilation. At least it is so for those who still see sanctity in Man as a potentially complete creature, even if we commonly see him as incomplete. I have concluded upon this point, because it is the crux of the controversy, in what I cannot pretend to be anything but a long series of controversies. Only I understand the crux better than when I began to controvert about it, ave crux, spes unica. We do not believe that man is a mass of mistakes that have to be shed until he has lost everything but shame in the very memory of his manhood. We think they are lower forms and fallen applications of his true powers and instincts, damaged by one great mystical mistake.
[GKC "Second Thoughts On Shaw" or "The Later Phases" CW11:603-4, written in 1934]
I feel it best that I add a note or two here...
Latin: Ave crux spes unica = "Hail, O Cross, the only hope". This is from the great hymn of the Passion, "Vexilla Regis Prodeunt" (The banners of the king come forth) by Venantius Fortunatus (530-609). [Hymns of the Breviary and Missal 123-6]. GKC here rebuts Shaw's heresy - which is still active in 21st century America - by expressing a sublime unity of Cross and Resurrection in what we might call Mystical Anthropology. Of course he expressed this elsewhere, as (for example) in one of my favourite quotes on this subject:Christianity spoke again and said: "I have always maintained that men were naturally backsliders; that human virtue tended of its own nature to rust or to rot; I have always said that human beings as such go wrong, especially happy human beings, especially proud and prosperous human beings. This eternal revolution, this suspicion sustained through centuries, you (being a vague modern) call the doctrine of progress. If you were a philosopher you would call it, as I do, the doctrine of original sin. You may call it the cosmic advance as much as you like; I call it what it is - the Fall."
[GKC Orthodoxy CW1:321]
Yes indeed: Man's "one great mystical mistake" really is "the Fall". But remember as GKC did (and Shaw did not) that the Church also calls it something else:
O certe necessarium Adae peccatum, quod Christi morte deletum est! O felix culpa, quae talem ac tantum meruit habere Redemptorem!
[from the "Praeconium Paschale" (the great hymn for the Vigil of Easter) in the Missale Romanum]
O truly needful sin of Adam, which was blotted out by the death of Christ! O happy fault, that merited to have such and so great a Redeemer!
[tr from St. Joseph Daily Missal]