My friend Graham Barlow, who lives in the ancient Roman resort town of Bath, has a wonderful podcast interview about the Origins of Aikido. It is a highly recommended walk on dark side.
This is an interview with Damon Smith. By the end, he argues that Aikido is in separable from the religion of Oomoto Kyo. If you want to read about my adventures in Japan with the Oomoto religion, read my book, Possible Origins, A Cultural History of Chinese Martial Arts, Theater and Religion. The part on Oomoto is in the introduction, where I discuss Japanese Tea Ceremony as a way to experience what a religion based in performing rituals is like.