One theme in this idyll is that it is too late to change what has been done. The young nun sings the following song:
The song describes the Lancelot/Guinevere affair. The couple thought they could meet one last time before departing and were caught by Modred. Then they think they can repent for their ignorance and all can be forgiven, but it is too late for that to happen. Lastly they think the can appeal to Arthur and have everything return to normal, but Arthur makes it clear that Guinevere can never return to Camelot:
Finally, Guinevere realizes too late the greatness of the King:
Thou art the highest and most human too,
It is too late to fix what has been done and Guinevere must live (though Arthur dies) with the consequences.
The above drawing is Arthur farewell to Guinevere (1867) by Gustave Dore.