[familyop]

The number, 25,000 (Catholics), is from John Carroll (bishop), 1785.

First federal census:

http://www.census.gov/prod/www/abs/decennial/1790.html

...or a little more or less from any one of many returns from a search.

[Verginius Rufus]

The original records for Virginia are lost--what they did was compile lists of names taken from tax records from the 1780s, which may not be as complete (and do not break down the white and black persons in each household into different categories based on age and sex). If I remember correctly there were some counties where they couldn't find any usable tax records. I think there are some other states where they had to resort to the same strategy in order to have anything at all.

Bishop Carroll's figure is from 5 years earlier and is presumably just a guess (although he would be better placed to make a guess than anyone else).