The Rabbinites had previously opposed Karaism in a guerilla warfare.

Saadiah came into the open, and met and vanquished the foe in pitched battles.

But he did more than defeat the invader, he strengthened the home defences.

Saadiah's polemical works have always a positive as well as a negative value.

He wished to prove Karaism wrong, but he also tried to show that Rabbinism was right.
As a champion of Rabbinism, then, Saadiah was called to Sura.