I would assume that when Hanks and Hodges make a statement like that they are referring to the United Kingdom, where they live, unless they specifically mention another country.
It is hard to get statistics by religion, but I would think the majority of men and boys named
Dominic in the United States still come from a
Roman Catholic background. The exception to that would be, as an earlier post states, its use in the African-American community. African-Americans are actually more religious on average than White Americans are, but of course the percentage of them who are
Roman Catholic is much smaller. Only about 6% of Black Americans are
Roman Catholic though that' still double the 3% who are
Muslim -- a great many Black Americans who have been given
Muslim names are not
Muslim themselves.