He is known for his theories about the Church of the East (at the time referred to inaccurately as "Nestorianism") and Jewish culture in Japan and for his involvement in the planning of the new Hiroshima after the atomic bombing.

Saeki was a Japanese Anglican who became an expert on the influence of the Church of the East, which at one time existed in China.

He was a close friend of Uchimura Kanzō, a Methodist evangelist, but their relationship turned sour later.

1908 he published a book in which he theorised the Hata clan, which arrived from Korea and settled in Japan in the third century, was a Jewish-"Nestorian" tribe.

After World War II, he was appointed mayor of Hatsukaichi in Hiroshima Prefecture.