Which preposition to use with proselyters

in Occurrences 1%

In one of the winter vacations of my course, my brother Paul, who was an ardent and sanguine proselyter in the Seventh-Day doctrine, charged me with an expedition up the Mohawk valley as a colporteur, to distribute Sabbath tracts, and, occasion arising, to discuss, with those who offered, the doctrine involved.

Which preposition to use with  proselyters