Actually, both tune and words predate Cat Stevens, and yes it is a hymnal song, although I don’t know when it was first written.
It’s #8 in the 1982 Episscopal [stet] Hymnal but not in the 1940 edition: Words: Eleanor Farejon (1881-1965) Music: Bunessan, Gaelic melody, harmonized by Alec Wyton (b. 1921)
Guess Cat and Vickie Jean will be humming it together soon.