The first Endowed to be identified are Farris channels.
These abilities are kept secret for generations because they are erratic, often come with massive health complications, and aren't much use for anything practical in those first centuries.
One reason channels become the focus of superstition is that it often seems to Gens that Simes can "read minds" -- or discern thoughts that have not been spoken. That idea is cause for shying away, avoiding contact, and sometimes spreading rumors of nefarious intentions. To keep the Householdings going, the Farris channels had to do everything they could to damp down such ideas. And so the entire range of Endowed abilities is denied and suppressed.
Mastering the Endowed abilities requires a channel to have a Companion who is both talented and trained in serving an Endowed channel.
Long after it became common knowledge that some few Farris channels exhibit these Psi talents, it is gradually determined that Endowment is occassionally (rarely) found among Gens and renSimes. It is usually dangerous and often fatal for Gens or renSimes to attempt to develop their talent, but eventually the genetic puzzle is unravelled and the tricks of the trade are codified to where, after the first contact interstellar wars, these scattered individuals can contribute the fruits of their talent to their communities and expect to survive the effort.