His father, Tamemitsu, placed high expectations on Sanenobu, and asked MINAMOTO no Tamenori, a renowned authority of literature at the time, to compile "Kuchizusami," a textbook for young aristocrats, and repeatedly implored his elder paternal half-brother FUJIWARA no Kaneie, who held the title of Sessho (regent), to make his son a Sangi.
父の為光も誠信の為に、当時の有名な文人源為憲に貴族の幼童用の教科書『口遊』の編纂を依頼したり、あるいはに異母兄の摂政藤原兼家に懇願を重ねて参議に就任させたりと、大いに将来を期待していた。