[QUOTE=Kimstu]
This may be just pushing your question back one step without actually answering it, but AFAIK most given names beginning with “St.” (always or almost always male names, I believe) are simply taken from last names beginning with “St.”
Giving boys first names that were originally surnames (e.g., Willis, Woodrow, Wesley, Wade, Ross, Riley, Ainslie, etc. etc. etc.) is a very common custom, and is currently being expanded to provide more girls’ first names as well (e.g., Mackenzie, Taylor, etc. etc.).
Now your question becomes “How did names of saints, including the prefix ‘Saint’, become common as surnames?” That I don’t know.
[/QUOTE]
I’m very much inclined to agree with Kimstu on the origin of most such names. There is a very common tradition to name children after saints, but without the “Saint…” tag. Frankie’s mother loved St. Francis of Assisi, Anne’s mother felt a devotion to the Virgin Mary’s mother, etc. But St. John Perse, not as a canonized John Perse, but as a member of the Perse family who was given the first name “St. John,” was probably named after his mother’s family, she having been the former Angela St. John or some such.
Then to essay what is not quite a WAG as to the origin of the “St.—” surnames but one for which I cannot adduce a cite, it’s my understanding that the majority (but not all) of them are toponymic in nature. The St. John family most likely lived in late-medieval times, when surnames became common, in the community that grew up around St. John’s Church and in consequence was the village of St. John. A very few of such surnames commemorate a married-and-father saint, duly canonized; this is far more common in the variants such as Gilchrist (servant of Christ) than with the actual Saint Something type names. As noted, there are such variants on the Saint-- surnames, Sinclair being the famous example (St. Clair > S’in’ Clair) – and these tend to be most commonly Scottish in origin.
