Several years before he undertook work on the new hymnal, Ralph Vaughan Williams made clear his opinion of the musicality of the 1861 Hymns Ancient and Modern; it was a very low opinion. He felt that the Salvation Army and the Lutherans had a far better grasp of the kind of music appropriate to sacred song; and that it ought to be drawn from the vigorous idiom of the people rather than from "the sentimental effusions of the Barnby school".

He was the obvious person to commission to revitalise the hymn repertoire with the kind of music that speaks directly to the heart (I speak here as a long-time atheist) and he did an excellent job of it. The fact that he was himself an atheist is irrelevant; unless, perhaps, his beliefs allowed him to use more imagination than a person hide-bound by "faith" might have been able to do.

I loved singing hymns at school, and most of my favourite tunes turned out to be those drawn from traditional sources. A lot of people feel the same way. Vaughan Williams did his job rather well, I'd say. Yes, I've read Julian Onderdonk's article. Had it been longer and spelled out in more detail, "Mo" might have had less difficulty with the concepts involved.

I expect Jim Carroll knows that the "Gaelic Psalm" style used to be common throughout Britain, though today it only survives in remote areas. There are some useful comments on that from Vic Gammon in thread  Gospel music is Gaelic?

Equally, it was the new fashion for organs and "continental" musical styles that drove the church bands and the old "West Gallery" singing out of the churches and into the chapels and the pubs (depending on personal taste). Sacred music -on the demotic level- suffered badly as a result, and that was something that Vaughan Williams wanted to put right. He produced a large number of minor works based on traditional music, for choirs, schools and churches, and saw this as part of his duty to the re-vitalisation of national culture. He also did the "big" stuff, of course; but his small works have been a lot more influential than many people realise.