The dating given here refers to the period where the Faithful can be considered an individual party or group within Númenórean society. The principles and policies the Faithful espoused went back to the beginnings of Númenor - indeed, they were the norm for much of its history - but it was only with the emergence of opposition in the form of the King's Men that the Faithful arose as a distinct political grouping.
Dating the end of the Faithful raises similar difficulties, because many of these people survived the Downfall. Elendil and his followers preserved the tradition, as did many others who had settled in Middle-earth before Númenor was lost, especially at the haven of Pelargir. With the demise of the King's Men, however, effectively all the Dúnedain in northwestern Middle-earth were descended from the Faithful, and there was little purpose in continuing to identify this group as a separate party.