Let's explore the context of that letter.

It's late 793. Alcuin of York, British Isles native and key advisor to Charlemagne, has just received word that a monastery in Northumberland--Lindisfarne--was raided by barbarians, many of its monks massacred or kidnapped as slaves. Lindisfarne Priory had a rich legacy: it had served as a center for evangelizing England, and its scribes produced works of art like the Lindisfarne Gospels. The local religious community is physically and emotionally shattered. Why is God punishing them so? Why did St. Cuthbert (buried at the monastery) and the other saints fail to protect them against the onslaught?

Alcuin senses a crisis of devotion in the making, and writes this letter to shore up the monks' courage and zeal for religious life. He explains:

It has not happened by chance, but is the sign of some great guilt...God chastises every son whom he accepts, so perhaps he has chastised you more because he loves you more.

This provides an explanation for the monks seeking a reason for the devastation: they haven't been good enough as monks. Alcuin then offers a prescription:

If anything needs correction in your way of gentleness, correct it quickly...Do not glory in the vanity of dress; that is cause for shame, not boasting, in priests and servants of God. Do not blur the words of your prayers by drunkenness. Do not go out after the indulgences of the flesh and the greed of the world, but stand firm in the service of God and the discipline of the monastic life, that the holy fathers whose sons you are may not cease to protect you.

Throughout the letter, building to a rousing conclusion, Alcuin's tone is one of consolation and imbuing courage:

Almost the whole of Europe has been denuded with fire and sword by Goths and Huns, but now by God's mercy is as bright with churches as the sky with stars and in them the offices of the Christian religion grow and flourish. Encourage each other, saying, "Let us return to the Lord our God, for he is very forgiving and never deserts those who hope in him."

And finally, Alcuin offers what the monks probably wanted most of all: a promise of concrete action.

When our lord King Charles [Charlemagne!] returns from defeating his enemies, by God's mercy, I plan to go to him, and if I can then do anything for you about the boys who have been carried off by the pagans as prisoners or about any other of your needs, I shall make every effort to see that it is done.

Alcuin wrote to encourage a very specific community of Christian clergy to hold fast to their way of life, even to improve it, in the wake of a catastrophic attack. Viking raids as a threat to Christianity in toto were never the issue, but Alcuin suspected they had absolutely torched the morale of the monks and priests who had lost their brothers to death and kidnapping.