Note a few things about this man and his writing.
1. His worldview is profoundly Christian - utterly and totally Christian (i. e., Catholic).
2. He has a clear-eyed even-handed vision of the reality of things as they are: fallen humanity, the workings of the Incarnation in a sinful world.
3. And yet he never loses sight of the ideal. He is able to look at things realistically without denigrating the ideal that things invariably fall shy of. And he is very fair to both.
... and from this fairness, one sees immense Charity.
Tolkien's letter continues with the story of his courtship of Michael's mother, and ends with his famous acclamation of the glories of the Blessed Sacrament.
You can read that part of it - indeed the whole thing - here.