> Yesterday I delivered food to a group of nuns that were labeled as hate group members by the SPLC. SPLC's justification was that a now-dead man had sold an anti-Semitic book back in the 1980s.

This seems highly improbable to be the sole basis for such a designation, and would be readily verifiable (or falsifiable, either way) if you identified the group, since the SPLC publishes details of the designated organizations and their activities that led to the designation online.