Although the funeral ceremony of Itagaki was conducted with Buddhist rites in accordance with Itagaki's will, as the religious doctrine of the Itagaki family was the Soto sect of Zen Buddhism, Itagaki himself was a Protestant and greatly affected Kenkichi KATAOKA (the first chairman of the House of Representatives) from the same village as Itagaki with regard to his experience in the religion.
板垣家の宗旨は曹洞宗であり、退助自身の葬儀は遺志により仏式で行われたが、退助自身はプロテスタントでもあり、同郷の片岡健吉（初代衆議院議長）の入信などに多大な影響を与えた。