Saadia is remembered as the greatest of the gaonim — those leaders of the Babylonian academies who added so much to the richness of Jewish life and thought. They were all great intellectuals, and Saadia, who flourished in the first half of the 10th century in Muslim Bagdad, was the greatest of them all.Later in the article we get a correct account of Saadia's relationship with the Karaites, which handily offsets the incorrect one in the article noted here.
Saadia’s main intellectual and spiritual opponents were the Karaites. The Karaites rejected the rabbinic principle that two Torahs were given to Moses on Mount Sinai: the Written Law and the Oral Law. The Karaites held that only the Written Torah was given at Sinai. This smaller Torah is the sacred text of the Torah scroll. The Oral Torah became the basis for the Talmud and halacha, which the Karaites held was not divine.It should be clarified that this reflects the situation in the tenth century and for quite some time thereafter, but not in the present. In the present, the Karaites are still alive and well, and the State of Israel accepts Karaites as legitimate Jews at least to the point of permitting them to make Aliyah (immigrate to Israel). Background on the Karaites is here, here, here, and links.
(The ancient Sadducees also rejected the twofold Torah in favor of just the Written Law.)
Saadia became the great champion of rabbinic normalcy and defeated this great threat to Talmudic Judaism.
Of the three major arguments for which he is known, two of them bore important fruit. The theologian’s battle against the Karaites prevented a serious schism within the Jewish community, allowing rabbinic Judaism to flourish into modern times.